The insertion of baffle plates in a duct is one of some countermeasures of the self-sustained tone generated from the duct with two tube banks. However the use of the baffle plate in the tube banks with cavity has unclear points in the baffle plate length and the insertion conditions. Then the aim of this study is to establish the best insertion condition of the baffle plate in the tube banks with the cavity. And the new idea has been verified by experiments that the suppression mechanism of the self-sustained tone is due to the decrease of the particle velocity and the Karman vortex. In the present experiments, the suppression effect of the baffle plate on the self-sustained tone has been examined. As a result, it was clarified that the insertion of the baffle plate only the tube bank is not effective for the suppression of the self-sustained tone in the case of plural banks with cavity. It was also clarified that the self-sustained tone disappeared when the baffle plate was inserted in and just down the tube bank where the fluctuation velocity was large.