PURPOSE: To lighten a device, and reduce clearances between each rotor and between rotors and a housing to obtain a supercharger having a high discharge efficiency by adjusting a coefficient of linear expansion at a specified value, and making rotors of the resin composition having a glass-transition temperature at a specified degree or more.

CONSTITUTION: As the resin composition for the material of rotors 4, 4 and screw rotors 14, 15, thermoplastic synthetic resin, which is mixed with inorganic or organic filling material and addition material to adjust a coefficient of linear expansion at 1.7E-5W2.7E-5/°C and which has a glass-transition temperature at 140°C or more and to which injection molding is applicable, is used. In ordinary, since aluminum alloy is used as the material of a housing 3 and 13, clearances between the housing and the rotors or between the screw rotors or between the rotors are maintained constant at the time of temperature rise with the operation. It is effective for preventing the collision to set the coefficient of linear expansion at the same degree with aluminium alloy, namely, at 1.7E-5W2.7E-5/°C by the blending of the filling material and the addition material, and the discharge efficiency is improved by setting the initial clearance at 0.1mm or less.