PURPOSE: To prevent a ring gear from coming off a fly-wheel by fixing the ring gear at the periphery of a fly-wheel by shrink-fitting and providing stoppers on both axial sides of the ring gear.

CONSTITUTION: A fly-wheel 2 is formed of an approximately disc formed drive plate 3 and a mass body 5. An approximately L-shape step part 6 is formed at the peripheral end of the mass body 5, and a ring gear 7 is fixed at this step part 6 by shrink-fitting. The flange part 9a of a clutch cover 9 functions as a stopper for regulating the axial movement of the ring gear 7 in cooperation with the raised wall 6a of the step part 6 formed at the mass body 5. Even if the ring gear is thermally expanded and its tightening force is loosened, the axial movement of the ring gear 7 is impeded by the stopper. The ring gear 7 is thus prevented from coming off the fly-wheel 2 to damage other parts.