PURPOSE: To prevent vibration of an input shaft, by a method wherein the center of gravity of a balance weight and that of a rotor are positioned facing each other with an input shaft positioned therebetween so as to effect well-balanced smooth rotation.

CONSTITUTION: The rotational force of an internal combustion engine is transmitted to an input shaft 15 through a belt 21 and a drive pulley 20, and if, a rotor 12 is thus turned, the open air is sucked through a suction hole 13, and it is exhausted through an exhaust hole 14, whereby connection of the exhaust hole 14 to the suction pipe of the internal combustion engine causes a device to work as a supercharger. In this supercharger, unlike a previous type, since a straight line (a), interconnecting the center of gravity 121 of the rotor 12 and the center of gravity 181 of a balance weight 18, is arranged in a positional relationship in which the line extends normally to the center of the input shaft 15, the rotor 12 and the balance weight 18 are easily well-balanced with each other, whereby smooth operation is effected, resulting in the possibility to prevent vibration of an input shaft 2.