To prevent the self-exciting vibration of a variable displacement piston pump during inverter drive.

A rotating speed to be commanded to an inverter device 30 is calculated in accordance with the detected pressure of a control piston 17 in a pressure chamber 18. When the calculated rotating speed is higher than a currently commanded rotating speed by a certain value d or greater, a value that the certain value d is subtracted from the calculated value is set as the rotating speed to be commanded to the inverter device 30. When the calculated rotating speed is lower than the currently commanded rotating speed by the certain value d or greater, a value that the certain value d is added to the calculated value is set as the rotating speed to be commanded to the inverter device 30. When an absolute value for a difference between the calculated rotating speed and the currently commanded rotating speed is the certain value d or smaller, backlash operation is performed not to change the value for the rotating speed to be commanded to the inverter device 30, and it is set as the rotating speed to be commanded to the inverter device 30.