by the rotor-housing 225a and a bearing-housing 225b, and a bypass passage for the intake air opens in the rotor-housing 225a. The lower shaft 221 shown in FIG. 5 is a driven shaft of the mechanical supercharger, and extends through the rotor-housing 225a so as to project into the side-housing 226a.
Within the side-housing 226a is a wet type multiple plate clutch 230 which may be conventional and which transmits the rotation of a pulley 228 to the shaft 221. A shaft 229 of the pulley 228 is driven by a direct driving system of the internal combustion engine. The pulley 228 and the shaft 229 are rotationally supported in the side-housing 226a by ball bearing 233, together with a supporting plate 232 supporting multiple plates 231 of thedriving side of the multiple plate clutch 230. To the shaft 221 is fixed a supporting plate 236 supporting multiple plates 235 of the driven side of the multiple plate clutch 230, and the multiple plates 235 are disposed between the multiple plates 231 of the driving side. The multiple plates 231, 235 rotate together with their respective supporting plates 232, 236 and are able to move axially thereon. The multiple plates 231, 235 are pressed into engagement for coupling the shafts 221 and 229 by the magnetic force resulting from excitation of coil 237 fixed to the side-housing 226a, and the force of this engagement is controlled by the amount of the excitation current supplied to the coil 237. Namely, the torque transmitted from the shaft 229