PURPOSE: To surely prevent leakage of oil and to simply achieve machining by a method wherein a groove extending in an axial direction is formed in a spline fit-in part between a cylinder block and a rotary shaft throughout the whole length of a fit-in part.

CONSTITUTION: In a fit-in hole 41 formed in the central part of a cylinder block 40, a spline groove 41a is formed in a part of the inner peripheral surface thereof and five grooves 45 extending in an axial direction are engraved at equal intervals in a peripheral direction more deeply than the spline groove 41a. A motor output shaft 31 has a protrusion fitted in the spline groove 41a of the cylinder block 40 and in a spline fit-in state, a communication hole extending along a motor output shaft 31 consists of the groove 45 axially formed in the inner peripheral surface of the fit-in hole 41. When oil leaking in the direction of a center flows to a motor output shaft 31, the five grooves 45 formed in a fit-in part between the motor output shaft 31 and the cylinder block 40 leads leakage oil to the protrusion side of a piston 50 to recover the leakage oil in a casing.