A driver (41) for driving a rotary motor (20) directly connected to a rotating shaft member of a supercharger is mounted on a board (39) together with a control device (51) and a heat sink (39a) and is housed in an engine room. Even when a temperature (ambient temperature) in the engine room detected by a temperature sensor (47b) rises up to an upper limit temperature (upper limit ambient temperature) of the ambient temperature, if a temperature (driver temperature) of a portion of the driver (41) that has a highest temperature detected by a temperature sensor (47a) is yet to rise to an upper limit temperature (upper limit internal temperature) of the driver temperature, the control device (51) causes the rotary motor (20) to continue to drive as the motor while lowering an upper limit of an output current.