We propose an Adjustable Parallel TCP (AP-TCP) which is a new scheme to control the aggregate throughput of parallel TCP flows. The AP-TCP can adjust the aggregate throughput to be any desired level irrespective of the parallel size (the number of parallel TCP flows). To adjust the aggregate throughput, we modify the increment factor of each parallel TCP flow to K²/N² where N is the number of parallel TCP flows and K is a value equivalent to any desired level for the aggregate throughput. Once K is given, the AP-TCP attempts to have K times more bandwidth than a single TCP flow when they are competing on the same network path. Another feature of the AP-TCP is its self-adjustment scheme. There is no central coordination or control overhead for parallel TCP flows. We analyze the model of the AP-TCP theoretically and evaluate it by using NS-2 simulation.