PURPOSE: To prevent an early abrasion of a piston head and a cam face, and a functional deterioration of a coupling, owing to oil shortage, by driving by the rotation speed difference between a rotor and a housing, and providing a pumping means to pour the oil in the outer side of the coupling to the clearance between the piston head and the cam face.

CONSTITUTION: When a rotor 40 rotates to a housing 30, the oil near a rotor shaft 40a is pushed to the outer side of the rotor 40 by a pumping function between spiral grooves 21a or 21b and the inner surface of the housing 30, it pushes to open a check valve 21d, and is poured between a piston head 52 and a cam face 31. Furthermore, when the oil is relieved from an accumulator chamber 90, an accumulator piston 91 strokes to the right exceeding the oil amount, and when a relief hole 42 is opened, the oil is poured from the relief hole 42 to the clearance between the piston head 52 and the cam face 31, through a communication groove 21c → the spiral groove 21a or 21b → the check valve 21d. Consequently, early abrasion of the piston head and the cam face 31 owing to an oil shortage can be prevented.