PURPOSE: To improve the self-sucking performance of a pump by tilting a cylinder port to the direction of circumference of the rotation of a cylinder block and by opening the port in such a manner that the center of the opening of the port on the side of a valve plate is in line with the center of the bore of a cylinder.

CONSTITUTION: A port 11 is titled to the back in relation to the rotational direction of a cylinder block 5, and the angle of tilt is set so as to accord with the tilt angle of the resultant velocity vector of the circumferential velocity of the rotation of the cylinder block 5 and the moving velocity of a piston 4. In his way, the flow-in of the working oil in the course of suction stroke of the piston 4 is made smooth, the suction resistance is reduced, and thereby the self-sucking capability can be increased. On the other hand, since the center O1 of the opening of the port 11 is made to accord with the center O2 of the bore of a cylinder 10, the discharge counterforces generated between a valve plate 6 and the cylinder block 5 in the course of discharge stroke of te pump and repulsive to each other do not become a couple of forces to cause the rotation in any directions in relation to the axial line of the cylinder. Thus, vibrations of the cylinder block 5 are held down and the stability of the operation of the pump is obtained.