An output of an internal combustion engine is transmitted to an output shaft by controlling a control motor to change a rotation speed output from the engine without speed steps via power distribution planetary gear. An output of a drive motor is input to the output shaft by changing the speed into two-step decelerated rotation by means of an automatic transmission. The transmission produces a desired torque by decelerating the speed by a large degree when a vehicle speed is low, and produces a desired number of revolutions by decelerating the speed by a small degree when the vehicle speed is high.