PURPOSE: To prevent the application of excessive force to the butted faces of a cylinder block and a valve plate, by providing the valve plate with recesses and accessory balance cylinders between the ends of an outlet and an inlet ports.

CONSTITUTION: The valve plate 6 is provided with the recesses 11 and the accessory balance cylinders 12 between the ends of the outlet and the inlet ports 9, 10. Oil pressure is introduced into the accessory balance cylinders 12 through the recesses 11. In thrusting the valve plate 6 by main balance cylinders 7 and the accessory balance cylinders 12, the auxiliary balance cylinders act when the number of the cylinders opened into the outlet port 9 is five. The main balance cylinders 7 are so provided that a minimum thrust force is applied to the butted faces 8 of the cylinder block and the valve plate when the number of the cylinders opened into the outlet port 9 is four. A shortage of the thrust force is eliminated by the accessory balance cylinders 12. The contact pressure on the butted faces 8 is thus prevented from becoming excessive.