PURPOSE: To improve drivability or fuel consumption by driving a mechanical supercharger via a rotational speed variation device and acceleration gear device by means of an internal combustion engine, and also controlling energizing force of springs in the rotational speed variation device by means of the actuator of a control device.

CONSTITUTION: A V belt 28 is wound across a driving pulley 34 and a driven pulley 22 in a rotational speed variation device 58, and a weight roller 27 and a coned disc spring 41 are arranged on one of both pulleys 22, 34, in a rotational speed variation device 18 which is driven by an internal combustion engine. A mechanical supercharger 1 is driven by the rotational speed variation device 58 via an acceleration gear device 3 so as to supercharge the internal combustion engine as well as a runner shaft 5 provided with a runner 4 rotatably supported is provided inside a housing 2. Additionally, an actuator 44 which controls energizing force of the coned disc spring 41 in a rotational speed variation device 58 is arranged in a control device 59. A cylinder 44a equipped with the coned disc spring 41 is arranged in the actuator 44.