To reduce a friction loss generated due to the contact between drive plates and driven plates upon releasing a compression by a piston.

The drive plates 1 are formed in a corrugated shape having a plurality of swells in the circumferential direction and inner peripheral edges and outer peripheral edges form dish configurations. Thus, under a state that the compression of the drive plates 1 and the driven plates 15 is released by the piston 19, the drive plates 1 come into point contact with the driven plates 15. Thus, the friction loss when the drive plates 1 and the driven plates 15 relatively rotate is reduced. Since the drive plates 1 have resiliency, a fastening shock upon fastening the drive plates 1 to the driven plates 15 can be decreased.