To light a weight so as to ensure sufficient rigidity and durability by forming a cylindrical part guided on the annular inner wall surface of a clutch cylinder in large and small two steps, and integratedly and continuously disposing an annular plate part along a diameter direction on the axial end of the small diameter cylindrical part.

Seal lips 30, 31 are fitted to an inner cylindrical part 19 and an outer cylindrical part 20 of a clutch piston 11 to ensure oil pressure for operating the clutch piston 11. A fastening margin is controlled, so as to be brought in linear contact or brought in surface contact a tip end of the lip with an outer peripheral surface of an inner cylindrical part 9 of the clutch cylinder 3 and an inner circumferential surface of the outer cylindrical part 10. In the clutch piston 11, the outer cylindrical part 20 guided on an annular inner wall surface of the clutch cylinder 3 is formed into large and small shapes of two steps. Since a step difference size between a small diameter part 21 and a large diameter part 22 in the outer cylindrical part 20 is rationalized, weight can be lightened, using a thin wall plate material, so that rigidity and durability can be ensured.