To prevent vibration and noise, etc., due to pulsation, to improve the efficiency and life of a pump.

One side of an oil hole 17, positioned on the movable locuses of respective cylinder ports 8A, is provided on one-side of a directional control valve part of a valve plate 14, and the other side of an oil hole 18 is provided on the other side of a directional control valve part. Also, a communication path 19 is formed on a rear casing 4 so that the oil holes 17 and 18 can be constantly communicated, and also a check valve 21 for allowing the flow of a pressurized oil from one side oil hole 17 to another side oil hole 18, and checking a reverse direction flow, is provided on the halfway of the path 19.