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.