There are provided an engine 1; a transmission 2 that transmits rotation of the engine 1 to drive wheels; a rotating electric machine 4; a reduction-ratio variator 6 transferring power by means of stepless speed change between the rotating electric machine 4 and the speed change transmission 2; an electric-storage element 12 storing electric power for supply to the rotating electric machine 4; and a control device for controlling the engine 1, the rotating electric machine 4 and the reduction-ratio variator 6 in accordance with operation conditions. The control device controls an output of the rotating electric machine 4 to provide the output required of the rotating electric machine 4 and also the rotational speed of the rotating electric machine 4 based on the optimum efficiency line of the rotating electric machine 4 so that the efficiency of the rotating electric machine 4 becomes optimal. The control device concurrently controls a reduction ratio of the reduction-ratio variator 6 in accordance with the vehicle speed at the time.