Sufficient pressure is produced in a pocket of a hydrostatic bearing when a corresponding cylinder is about to switch from a suction stroke to a discharge stroke in order to prevent solid contact of sliding faces. A plurality of cylinders are arranged on a circle around a center axis of a cylinder block rotatably supported in a housing. Pistons reciprocates in the respective cylinders. A suction port and a discharge port provided to the housing selectively communicate with the respective cylinders in accordance with a rotational position of the cylinder block. A drive shaft inclined relative to the center axis of the cylinder block, a rotation transmitting mechanism for transmitting rotation of the drive shaft to the cylinder block for rotating the cylinder block synchronously with the drive shaft, and a rotating disk rotating together with the drive shaft and cooperatively engaged with the pistons are further provided. A fixed sliding contact face is formed in the housing to have a sliding contact with a rear face of the rotating disk. Pressure pockets are formed on the rear face of the rotating disk in correspondence with positions of the respective pistons. Pressure paths for conducting hydraulic pressure in the respective cylinders to the respective pressure pockets via the pistons are further provided. There is further provided a pressure introducing mechanism for introducing a high pressure fluid to a pocket moving along the fixed sliding contact face along with the rotating disk at a position in which a corresponding cylinder is about to switch from a suction stroke to a discharge stroke.