Piston

A part of a rubber layer of the base section of a piston seal is replaced with a reinforcement ring with high rigidity, and a part of the reinforcement ring reaches the root of a seal lip. This provides the piston seal with required rigidity. Further, providing an outer diameter side radial section on a piston body secures a contact width coming into contact with a section to be pressed. A piston has a piston body ( 111 ) having a clutch pressing section ( 111 f) provided so as to be in surface-contact with a multiple disc clutch ( 105 ), a reinforcement ring ( 112 ) fitted on the outer periphery of an outer cylinder section ( 111 e) of the piston body, and an outer peripheral seal lip ( 113 a) provided on the reinforcement ring and in sealing contact with the inner peripheral surface ( 102 a) of the outer peripheral section of a housing ( 102 ).