To reduce the drag torque by forming a wet fcitional member by an annular flat body, and providing the member with a plurality of grooves penetrating from an inner peripheral edge of a surface opposite to a surface fixed to a frictional plate to an outer peripheral edge, and having the even number of bent parts on the way.

A plurality of grooved parts 81 is formed in the circumferential direction on a surface of a first frictional member 46a, the intervals therebetween may be uneven, the hydraulic fluid flows in the radial direction in the connection of a clutch for cooling a frictional surface, and the pressure holding effect can be obtained in the disconnection. The grooved part 81 has too bent parts 85, 86, and formed by a first grooved part 82, a second grooved part 83 and a third grooved part 84, the outer peripheral side ends of the grooved parts 82, 83 are positioned at R2 side in comparison with the inner peripheral side ends, and the grooved part 84 is approximately straightly extended. Whereby the hydraulic fluid mainly flows from the grooved part 82 at the frictional member 46a side, and the hydraulic fluid collides with the bent part 85, by which the pressure is increased and the dynamic pressure is generated. The frictional connecting part of a piston is energized from a frictional surface of a front cover to an axial transmission side, as a result, the drag torque is scarcely generated.