JP3119506B2 - Cooking device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cooking device provided with a gas sensor.

[0002]

2. Description of the Related Art In recent years, heating cookers which perform various types of cooking in a comprehensive manner have become mainstream. For example, in a microwave oven, an oven function using a heater, etc., in addition to a microwave function. Many are equipped with.
This type of cooking device is capable of baking bread and the like, and has a so-called toaster function.

[0003] Although bread is usually baked from a room temperature, bread is often stored in a freezer compartment of a refrigerator, and the frozen bread (hereinafter referred to as frozen bread) is used. (Hereinafter referred to as room temperature bread) requires a different heating control pattern (for example, heating time) from the case of baking bread at normal temperature (hereinafter referred to as room temperature bread). For this reason, this kind of cooking device is provided with a normal temperature bread designation switch and a frozen bread designation switch for toaster cooking, so that the user can select and designate the bread state in advance, and based on the selection designation input, the heating is performed. The heating operation control device of the cooker switches the heating control pattern.

[0004]

However, as described above, in this type of cooking device, since the mainstream is to perform various types of cooking in a comprehensive manner, the number of input switches is large. Therefore, it is preferable not to increase the number of switches as much as possible. From this viewpoint, there is a strong demand to eliminate the normal temperature bread designation switch and the frozen bread designation switch. Further, when the pan state is selected and designated by the switch operation by the user in this way, the switch operation may be mistaken,
There is a risk that cooking will fail.

[0005] In addition, when baking bread, since the bread is generally in the initial state often at room temperature, usually the heating output is set so as to be suitable for baking room temperature bread. However, in the case of a frozen bread, the inside of the bread may not be so hot even when the bread surface is in an appropriate baking color. If the heating output is increased to heat the inside of the bread, the bread surface may be over-baked.

[0006] Furthermore, as described above, the initial state of the bread is a normal temperature bread or a frozen bread. However, depending on the user, the bread is stored in a refrigerator room other than the refrigerator room instead of the refrigerator room. Sometimes. Therefore, if the bread is divided into the normal temperature bread and the frozen bread, the intermediate refrigerated bread is heated by the control for the normal temperature bread or the control for the frozen bread, which is not preferable.

[0007] The present invention has been made in view of the above circumstances, and an object of the present invention is to automatically determine the initial state of a food to be heated, reduce the number of switches, and
An object of the present invention is to provide a heating cooker that does not cause a cooking failure due to a switch erroneous operation.

It is another object of the present invention to automatically determine the initial state of the object to be heated, to reduce the number of switches, not to cause a cooking failure due to an erroneous switch operation, and to change the initial state. It is still another object of the present invention to provide a heating cooker that can always cook well.

It is a further object of the present invention to automatically determine the initial state of the object to be heated, to reduce the number of switches, not to cause a cooking failure due to an erroneous operation of the switch, and in particular to the cooking of the object to be heated. An object of the present invention is to provide a heating cooker that can determine an initial state relatively finely.

[0010]

The present invention has been made by focusing on the following points. When the heated food is heated,
It has been found that the output state of the gas sensor differs depending on the initial state. That is, when the initial state of the food to be heated is distinguished into a normal temperature state and a frozen state, when the food to be heated in the normal temperature state is heated, steam or gas is immediately generated from the food to be heated. When the food to be heated is heated, it takes a long time for the solidified water contained in the food to be heated to melt, and the generation of steam and gas is delayed.

[0011] However, despite the difference in the gas generation situation, there is not much difference in the detection output of the gas sensor. When investigating this point, it was found that there is no difference in the output of the gas sensor until it reaches a certain amount of gas, and that if the gas contact degree of the gas sensor is small, there is no difference in the output even if the amount of generated gas is large. found.

[0012] The heating cooker of the present invention comprises a heating means for heating an object to be heated accommodated in a cooking chamber, a blower for supplying and discharging air into the cooking chamber, and detecting a gas amount in the cooking chamber. A gas sensor, an inspection control unit that drives the blower after performing the inspection operation for driving the heating unit for a predetermined time while the blower is stopped, and an inspection control unit that drives the blower after the inspection operation. The present invention is characterized in that it comprises a judging means for detecting a gas amount by a gas sensor and judging an initial state of the article to be heated in accordance with a difference between the detected gas amounts (the invention of claim 1).

Further, the heating cooker of the present invention is characterized in that a heating output control means for controlling a heating output of the heating means in accordance with the result of the judgment by the judging means is added to the constituent elements of the first aspect of the present invention. It has features (the invention of claim 2).

Further, in the cooking device according to the present invention, instead of the judging means of the first aspect, before and after the end of the inspection operation, a gas sensor detects a gas amount, and a difference between the detected gas amount values is determined in advance. The present invention is characterized in that a judging means for judging the initial state of the object to be heated to one of three categories by comparing with the reference value obtained is provided (the invention of claim 3).

[0015]

When the inspection operation is performed, the food to be heated is heated. In this case, since the blower is stopped, if the object to be heated is in a normal temperature state, gas is sequentially generated and accumulates in the cooking chamber. In addition, if the food to be heated is in a frozen state, almost no gas is generated.

Before the end of the inspection operation, the gas amount is detected by the judging means. At this time, the gas amount detection result is not so different whether the object to be heated is in a normal temperature state or a frozen state. When the inspection operation is completed, the blower is operated. Then, the gas amount is detected again.

At this time, if the object to be heated is at a normal temperature, a considerable amount of gas is accumulated in the cooking chamber in the inspection operation, and the gas comes into good contact with the gas sensor by the blowing operation of the blower. Then, the gas sensor detects a considerable amount of gas at a time, and there is a large difference from the first gas amount detection result. With this, it is determined that the object to be heated is at the normal temperature. on the other hand,
Assuming that the cooked food is in a frozen state, since the amount of gas generated is extremely small, there is almost no difference from the detection result at the time of the first gas amount detection, which indicates that the cooked food is in a frozen state. Is determined. In this way, the initial state of the object to be heated can be determined automatically and without error, so that subsequent cooking can be performed appropriately. Therefore, a switch for designating the initial state of the food to be heated is unnecessary, and the number of switches can be reduced, and cooking failure due to erroneous switch operation does not occur.

In this case, if the heating output control means for controlling the heating output of the heating means in accordance with the judgment result of the judging means is provided, the object to be heated can be cooked with the heating output corresponding to the initial state. Therefore, even when the initial state is different, it is possible to always cook well.

Further, the judging means detects a gas amount by a gas sensor before and after the end of the inspection operation, compares the difference between the detected gas amount with a predetermined reference value, and compares the difference between the detected gas amount with a predetermined reference value. Is determined to be one of the three categories, the initial state of the cooked food can be determined relatively finely.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention will be described below with reference to FIGS. 2 and 3, an inner case 2 is provided inside a main body case 1, and the inside of the inner case 2 is a cooking chamber 3. A door 4 is provided on the front surface of the main body case 1 and an operation panel 5 is provided.

The operation panel 5 is provided with a display 6 and various switches 7 such as a start switch, an automatic cooking switch, a toaster switch, and a time setting switch.

A turntable motor 9 for rotating the turntable 8 is provided on the outer surface of the bottom plate of the inner case 2, and a lower heater 10 as heating means is provided on the inner case 2. An upper heater 11 is also provided on the outer surface of the plate portion as heating means. Further, an excitation port 12 is formed in a side plate portion of the inner case 2, and a magnetron 14 is attached to the excitation port 12 via a waveguide 13. The side plate of the inner case 2 has an exhaust port 2a and an exhaust passage 2b. And inside this exhaust passage 2b,
A gas sensor 15 is provided. This gas sensor 15
Is configured to output the gas amount detection as a voltage, and the output voltage decreases as the detected gas amount increases.

A fan motor 1 is provided on the inner surface of the main body case 1.
A blower 18 including the blades 6 and the blades 17 is provided. When the blower 18 is operated, outside air is sucked from an intake port (not shown) of the main body case 1, and the magnetron 14 is air-cooled into the cooking chamber 3. Inlet and outlet 2a
Then, the air is discharged outside the main body case 1 through the exhaust passage 2b.

FIG. 1 shows an electrical configuration. The operation control circuit 19 includes an A / D converter and a microcomputer. The operation control circuit 19 includes a turntable motor 9 and a lower heater 1 according to an operation program stored therein.
The drive control of the display unit 6 is performed while the drive control of the heater 0, the upper heater 11, the magnetron 14 and the fan motor 16 is performed via drive circuits 20 to 24, respectively. This operation control circuit 19 also functions as inspection control means and determination means.

Now, the operation of the above configuration will be described together with the respective functions of the operation control circuit 19. 4 and 5 show flowcharts of the control contents of the operation control circuit 19. This flowchart starts when the toast switch is operated. Also, in FIG.
The output change of the gas sensor 15 when heating and cooking a normal temperature pan (this is indicated by a symbol J) and the output change of the gas sensor 15 when heating and cooking a frozen pan (this is indicated by a symbol R) are determined by the blower 18 and the lower part. , And upper heaters 10 and 11 are also shown.

Now, in order to bake the bread, the bread is put in the cooking room 3.
When the user operates the toast switch (when cooking starts) after being housed in the inside, the operation control circuit 19
First, a refresh operation is performed for a predetermined time, for example, 5 seconds from the start of cooking (step S1). This refresh operation is performed to drive the blower 18 to discharge the residual gas inside the cooking chamber 3. Next, the blower 18 is stopped, and an inspection operation for continuously driving the lower heater 10 and the upper heater 11 is started (step S2). Then, a gas amount detection value by the gas sensor 15, that is, a sensor output voltage is read (step S3), and the sensor output voltage is stored as a reference value Vmax (step S4). At this point, the detected gas amount of the gas sensor 15 is low (the output voltage is high) regardless of whether the bread is a normal-temperature bread or a frozen bread.

The inspection operation is performed in step S described later.
12 shows that a predetermined time, for example, 30 seconds (cooking
The inspection is performed until 35 seconds elapse , but before the end of the inspection operation, for example, 28 seconds from the start of the inspection operation (cooking).
When 33 seconds have passed since the start (step S)
5), read the sensor output voltage (step S
6) And the output voltage Vs at this time and the reference value Vm
calculating the difference between _{A 1} and ax (step S7). One second has passed since this time, ie 29 seconds from the start of inspection operation
Where (from the cooking start 34 seconds) has elapsed (decision at step S8), and reads the sensor output voltage (step S9), and then calculates a difference A ₂ between the output voltage Vs and the reference value Vmax in this case ( step S10), and further calculates the average value a of the difference _{a 1} and the difference _{a 2} (step S11). Then, one second after this, that is, when 30 seconds have elapsed since the start of the inspection operation (35 seconds after the start of cooking) (determined in step S12), the inspection operation is terminated.

At the time of the inspection operation, if the bread is a normal-temperature bread, the water inside is evaporated by the heating action of the upper and lower heaters 11 and 10. Therefore, the gas amount inside the cooking chamber 3 increases, but the gas contact degree with the gas sensor 15 remains low, so that the sensor output voltage remains high and changes little (see the characteristic line J in FIG. 6). Further, if the bread is a frozen bread, even if the bread is subjected to the heating action of the upper and lower heaters 11 and 10, only the internal coagulated water is melted and the generation of steam and other gases is extremely small, and from the characteristic line R in FIG. As can be seen, the output voltage of the gas sensor 15 does not change much.

After this inspection operation, the blower 18 is driven (step S13 in FIG. 5). Thus, the air in the cooking chamber 3 is discharged, and when the bread is a frozen bread, the output voltage of the gas sensor 15 does not change so much.
When the bread is a normal-temperature bread, a considerable amount of gas has already accumulated in the cooking chamber 3, and this gas comes into good contact with the gas sensor 15 by the blowing action of the blower 18.

Immediately after the start of the operation of the blower 18 (3 seconds later), that is, 33 seconds from the start of the inspection operation (cooking start or not)
38 seconds) (determined in step S14), the sensor output voltage Vs is read (step S1).
5) and the output voltage Vs at this time and the reference value Vm
calculating the difference between _{B 1} and ax (step S16). 1 second after this, ie 34 seconds from the start of inspection operation
When 39 seconds have passed since the start of cooking ( determined in step S17), the sensor output voltage Vs is read again (step S18), and the output voltage V at this time is read.
s and calculating the difference between _{B 2} and the reference value Vmax (step S19), and calculates an average value B of the difference _{B 1} and the difference _{B 2} (step S20).

Here, as can be seen from the above, if the bread is a room temperature bread, the output voltage of the gas sensor 15 drops (the detected gas amount is large), so that the reference value Vmax is used.
The average value B of the difference from the above becomes large, and this average value B does not increase so much when the bread is a frozen bread. Thereafter, the previously calculated average value A is subtracted from the average value B, and if the difference is equal to or larger than the reference value K (step S
21), it is determined that the bread is a normal temperature bread (step S).
22) If it is less than the reference value, it is determined that the bread is a frozen bread (step S23).

Then, for example, 90 seconds (control) from the start of the inspection operation.
When 95 seconds have passed since the start of the process (step S24), the blower 18 is stopped (step S25), and the remaining cooking time is set according to the result of the determination (step S26). Thereafter, when the set time has elapsed (determined in step S27), the heaters 10 and 11 are turned off (step S28), and the cooking operation is terminated.

According to this embodiment, it is possible to automatically and without error determine whether the bread to be heated is a normal-temperature bread or a frozen bread, so that the subsequent cooking can be performed properly. Becomes possible. Therefore, unlike the case where the initial state of the bread is specified by the user's switch input, the number of switches can be reduced and cooking failure such as overbaking or underbaking does not occur.

In the above embodiment, after the initial state of the bread is determined, the blower 18 is turned on for 90 seconds from the start of the inspection operation.
The stop is performed when (95 seconds have elapsed from the start of cooking) , but this may be stopped immediately after the determination.

FIGS. 7 to 11 show a second embodiment of the present invention. The operation control circuit 31 shown in FIG.
In addition to having the same functions as the inspection control means and the judgment means similar to those of the operation control circuit 19 of the embodiment, it also has a function as a heating output control means. The control contents of the operation control circuit 31 are shown in FIGS.
In the figure, steps P1 to P25 correspond to steps S1 to S25 in the first embodiment.
Same as 25. Up to this step 25, the upper and lower heaters 11, 10 are continuously energized, and it is determined whether the bread is a normal temperature bread or a frozen bread.

In step P26, it is determined whether the result of the determination is a frozen bread or not. If the bread is not a frozen bread (if it is a normal temperature bread), an appropriate remaining cooking time for the normal temperature bread is set (step P26). P27). In this case, the upper and lower heaters 11 and 10 are in a continuous energized state. If the determination result is a frozen pan ("N" in step P26), the upper and lower heaters 11 and 10 are turned off at a predetermined duty ratio (20-second energization, 10-second interruption) (step P28). ). That is, the upper and lower heaters 1
The heating output of 1, 10 is set to 100% output in the case of the normal temperature pan, and is set to approximately 67% output (conduction time ratio) in the case of the frozen pan.

After step P28, an appropriate remaining cooking time for the frozen pan is set (step P29). The remaining cooking time is set longer than in the case of the normal temperature pan. Thereafter, if the set time elapses (step P3
0), the upper and lower heaters 11 and 10 are turned off (step P31), and the cooking operation is terminated. FIG. 11 shows the power cut-off operation status of the upper and lower heaters 11 and 10.

According to the second embodiment, the following effects can be obtained. That is, when the upper and lower heaters 11 and 10 are energized at 100%, when baking the room temperature bread, the surface is burned to the extent that an appropriate baking color is obtained and the inside becomes hot, and a good baking condition can be expected. Even in such a case, if the same heating output is used, the inside may be relatively cold even if the bread surface is burned to an appropriate baking color. In the second embodiment, the heater output is set to 100% when it is determined that the pan is the normal temperature pan. However, when it is determined that the pan is the frozen pan, the heater output is controlled to approximately 67% in terms of the energization ratio. From that, the temperature rise on the bread surface is reduced, so the temperature difference with the inside is reduced, so the inside is also hot when the surface is baked to the extent that the appropriate baking color arrives,
Good baking condition can be expected. Therefore, whether the bread to be heated is a normal-temperature bread or a frozen bread, it is possible to always cook well.

FIGS. 12 to 15 show a third embodiment of the present invention. The operation control circuit 41 shown in FIG. 12 has the same functions as the inspection control means and the judgment means as the operation control circuit 19 of the first embodiment, but the configuration of the judgment means is slightly different. In the third embodiment,
A weight sensor 42 for detecting the weight of the food to be heated is provided, and the detection result is given to an operation control circuit 41.

The control content of the operation control circuit 41 is shown in FIG.
14 and FIG. 14, and the control contents will be described below with reference to FIG. FIG. 15 shows an output change of the gas sensor 15 when heating and cooking a normal-temperature pan (this is indicated by a symbol J), and an output change of the gas sensor 15 when heating and cooking a frozen pan (this is indicated by a symbol R). The change in the output of the gas sensor 15 (which is indicated by the symbol M) when heating and cooking bread (hereinafter referred to as “intermediate bread”) stored in a refrigerator or the like of a refrigerator is described with a blower 18 and the lower and upper heaters 10.
11 is also shown.

When the cooking is started, first, the weight W is read by the weight sensor 42 (step R1), and then the refresh operation is executed for 5 seconds (step R2). Thereafter, the blower 18 is stopped and the lower heater 1 is turned off.
An inspection operation for continuously energizing the 0 and upper heaters 11 is started (step R3). And this inspection luck
40 seconds from the start of rotation (45 seconds have passed since the start of cooking)
During this period, the gas amount detected by the gas sensor 15, that is, the sensor output voltage is read (step R 4), and the maximum value Vmax of the sensor output voltage is stored (step R 5). At this point, the detected gas amount of the gas sensor 15 is low (the output voltage is high) regardless of whether the bread is a normal temperature bread, a frozen bread, or an intermediate bread.

Then, 40 seconds from the start of the inspection operation (the cooking
When 45 seconds have passed since the start (step R6), the blower 18 is driven (step R7). And
55 seconds from the start of the inspection operation (60 seconds from the start of cooking )
Is passed (determined in step R8), the sensor output voltage A is read (step R9). Then, the change rate H of the gas detection amount is calculated (step R10). This change rate H
Is determined by H = (Vmax−A) / Vmax, which is replaced by H = 1−A / Vmax. That is, the difference in the gas detection amount is reflected in this.

Next, this rate of change H is compared with a preset reference value, in this case "0.08" and "0.03" (step R11, step R12). If the change rate H is equal to or more than "0.08", it is determined that the bread is a normal temperature bread (step R13). If the change rate H is less than "0.03", it is determined that the bread is a frozen bread (step R15).

Thereafter, the number of breads is determined based on the weight W read in step R1 (step R1).
6, Step R17). If the weight W is equal to or less than “70 g”, it is determined that “one sheet” (step R18), and the weight W is “7”.
If the weight W exceeds 0 g and is less than 140 g, it is determined to be “intermediate” (step R19), and if the weight W is not less than 140 g, it is determined to be “two” (step R20). The above “intermediate” is a determination level such that the number of sheets may be one or two depending on the thickness of the bread.

Next, 115 seconds from the start of the inspection operation (cooking
When 120 seconds have passed since the start (step R21)
And the blower 18 is stopped (step R22),
The cooking time (time from the start to the end of cooking ) is set (step R23). This cooking time setting is set based on the initial state of the bread and the determination result of the number of breads. Table 1 shows the contents of the setting data.

[0046]

[Table 1]

Thereafter, when the set time has elapsed (determined in step R24), the heaters 10 and 11 are turned off (step R25), and the cooking operation is terminated.

According to the third embodiment, it is possible to automatically determine whether the initial state of the bread is a normal-temperature bread, a frozen bread or an intermediate bread. Therefore, if the bread is stored in, for example, a refrigerator, it can be determined as an intermediate bread that is neither a room temperature bread nor a frozen bread. As described above, the initial state of the bread to be heated can be relatively finely determined, and thus the heating control can be optimally performed according to the initial state of the bread.

Particularly in the third embodiment, the weight sensor 4
The number of breads is also determined based on the detection result of No. 2, and the cooking time is set in consideration of the result of the determination of the number of breads in addition to the result of the initial state determination, so that the bread can always be satisfactorily baked.

[0050]

As apparent from the above description, the present invention has the following effects.

That is, according to the heating cooker of the first aspect, a heating means for heating the article to be heated accommodated in the cooking chamber, a blower for supplying and discharging air into the cooking chamber, and the cooking chamber A gas sensor for detecting the amount of gas, inspection control means for driving the heating means while the blower is stopped for a predetermined time and driving the blower after the inspection operation, and before the end of the inspection operation. And a determination means for detecting the gas amount by the gas sensor after the end and determining the initial state of the food to be heated in accordance with the difference between the gas amount detection values. As a result, the initial state of the object to be heated can be determined automatically and without error, so that subsequent cooking can be performed properly. Therefore, the initial state of the object to be heated can be determined by the user. Unlike when specifying at switch input, nor cause the cooking failure by the switch operation error with possible to reduce the switch number, exhibits the excellent effect that.

According to the second aspect of the present invention, in addition to the above-described components, the heating cooker is provided with heating output control means for controlling the heating output of the heating means in accordance with the determination result of the determination means. The cooked food can be cooked with a heating output corresponding to the initial state, and therefore, there is an effect that the cooked food can always be satisfactorily cooked even if the initial state differs.

According to the cooking device of the third aspect, the cooking device of the first aspect.
Instead of the determination means, the gas amount is detected by a gas sensor before and after the end of the inspection operation, and the difference between the gas amount detection values is compared with a predetermined reference value to determine the initial state of the object to be heated by three. Since the determination means is provided in one of the categories, the initial state of the object to be heated can be determined relatively finely, so that subsequent cooking can be performed more appropriately. It has the effect.

[Brief description of the drawings]

FIG. 1 is a block diagram of an electrical configuration showing a first embodiment of the present invention.

FIG. 2 is a perspective view of a microwave oven.

FIG. 3 is a vertical rear view.

FIG. 4 is a flowchart showing control contents.

FIG. 5 is a flowchart showing control contents.

FIG. 6 is a diagram showing an output change of a gas sensor and an operating state of a blower and a heater when the initial state of the bread is different.

FIG. 7 is a block diagram of an electrical configuration showing a second embodiment of the present invention.

FIG. 8 is a flowchart showing control contents.

FIG. 9 is a flowchart showing control contents.

FIG. 10 is a flowchart showing control contents.

FIG. 11 is a diagram showing a change in output of a gas sensor and an operating state of a blower and a heater when the initial state of the bread is different.

FIG. 12 is a block diagram of an electrical configuration showing a third embodiment of the present invention.

FIG. 13 is a flowchart showing control contents.

FIG. 14 is a flowchart showing control contents.

FIG. 15 is a diagram showing an output change of a gas sensor and an operating state of a blower and a heater when the initial state of the bread is different.

[Explanation of symbols]

3 is a cooking room, 10 is a lower heater (heating means), 11 is an upper heater (heating means), 14 is a magnetron, 15 is a gas sensor, 16 is a fan motor, 18 is a blower, 19 is an operation control circuit (inspection control means, Judgment means) and 31 indicate an operation control circuit (inspection control means, judgment means, heating output control means). Reference numeral 41 denotes an operation control circuit (inspection control means, determination means), and reference numeral 42 denotes a weight sensor.

Claims (3)

(57) [Claims] 1. A heating means for heating an object to be heated housed in a cooking chamber, a blower for supplying and discharging air into the cooking chamber, a gas sensor for detecting an amount of gas in the cooking chamber, and the blower Inspection operation for driving the heating means in a stopped state for a predetermined time and inspection control means for driving the blower after the inspection operation, and detecting the gas amount by the gas sensor before and after the end of the inspection operation, respectively. Determining means for determining an initial state of the article to be heated according to the difference between the detected gas amounts. 2. A heating means for heating an object to be heated housed in a cooking chamber, a blower for supplying and discharging air into the cooking chamber, a gas sensor for detecting an amount of gas in the cooking chamber, and the blower Inspection operation for driving the heating means in a stopped state for a predetermined time and inspection control means for driving the blower after the inspection operation, and detecting the gas amount by the gas sensor before and after the end of the inspection operation, respectively. Determining means for determining an initial state of the food to be heated in accordance with the difference between the detected gas amounts; and heating output control means for controlling a heating output of the heating means in accordance with the determination result of the determining means. A cooking device characterized by comprising: 3. A heating means for heating an object to be heated housed in a cooking chamber, a blower for supplying and discharging air into the cooking chamber, a gas sensor for detecting an amount of gas in the cooking chamber, and the blower. Inspection operation for driving the heating means in a stopped state for a predetermined time and inspection control means for driving the blower after the inspection operation, and detecting the gas amount by the gas sensor before and after the end of the inspection operation, respectively. Determining means for comparing the difference between the detected gas amounts with a predetermined reference value to determine the initial state of the object to be heated into one of three categories. .