PURPOSE: To provide a mechanical supercharger which prevents a supercharger from rotating in a state that the bearing of an impeller shaft is rendered short in oil, simplifies constitution, and is reduced in the weight and cost.

CONSTITUTION: A mechanical supercharger 1 comprises a supercharger 5 driven by an engine through a speed-increasing mechanism 3, an engine-driven oil pump 93 to feed oil to a bearing 71 which is designed to support the impeller shaft 57 of the supercharger 5, a clutch 7 located between the engine and the speed-increasing mechanism 3, and an actuator 9 to intercouple the oil pump 93 and the hydraulically-operated clutch 7.