A control system for a hybrid drive unit wherein an assist prime mover is connected through a transmission mechanism to an output member to which a torque outputted by a main prime mover is transmitted, a torque of said assist prime mover is limited within a preset range. Specifically, said torque is limited to an upper limit side in case the speed of said assist prime mover exceeds the preset value, and a lower limit value of the torque of the assist prime mover is set in case the output torque of said assist prime mover acts as a driving torque.