PURPOSE: To decrease rotary vibration by arranging a sun gear, provided in an accelerating part of drive force, in one side of an impeller shaft, and providing a rotary unit, fixing a supercharger impeller in the other side, and a bearing by which self centering can be performed by swivelably supporting the rotary unit in a sun gear side from its center of gravity.

CONSTITUTION: A belt type continuously variable transmission 3 of a mechanical supercharger 1 has a drive pulley 9 and a driven pulley 11, to transmit drive power to an accelerating part 5. The accelerating part 5 has an internal gear 35 and a sun gear 37 formed in the left end of an impeller shaft 39 of a supercharger 7. The impeller shaft 39 is supported by a bearing 49 in a point Q adjacent to a sun gear 37, and an impeller 53 is press fitted to the right end, to constitute a rotary unit 61. The center of gravity of the rotary unit 61 is placed in a position of an arrow mark 81, to swivelably support the bearing 49 with the point Q serving as the center. Since the sun gear 37 is adjacent to the bearing 49, small amplitude is generated. Since the impeller 53 of large mass is rotated by self centering, very small vibration is generated.