The invention makes an output sharing of an engine and an electric motor proper so as to suitably drive a load. When a throttle opening degree variation amount ΔV is within a ΔV1, an engine E can drive a power generator G at a rated revolution speed. A switching circuit 28 is switched in such a manner as to charge a battery 27 by a generated power. When the opening degree variation amount ΔV becomes larger than the ΔV1, a power assist of operating the power generator G as the electric motor is set to be standby. When the opening degree variation amount ΔV is more than ΔV2 (>ΔV1), the power assist is started. In the power assist, an angle advance amount is increased. When acceleration is insufficient, it is possible to further increase a conduction angle in stages.