PURPOSE: To smoothly reduce the vibration in a wide range by increasing the allowable relative turning angle between an input member and an output member.

CONSTITUTION: A ring gear 6 is installed on a piston plate 5 as input member. An external gear 14 is installed on a hub plate 9 as output member. A planetary gear 15 is meshed with the ring gear 6 and the external gear 14, and the planetary gear 15 is supported on a drive plate 16. A torsion spring 19 is interposed between the drive plate 16 and the hub plate 9, and both are connected elastically by the spring 19. The ring gear 6, external gear 14, planetary gear 15, and the drive plate 16 constitute a planetary gear mechanism. Since the relative turning angle between the drive plate 16 and the hub plate 9 is smaller than the relative turning angle between the piston plate 5 and the hub plate 9, the deflection quantity of the torsion spring 19 per turning angle is reduced.