PURPOSE: To maintain the operation and speed of an actuator constant preventing it from being affected by the change of oil temperature and load fluctuation by feeding the correction obtained by detecting the actual speed of the actuator back to the input side.

CONSTITUTION: An actuator's target speed command 5 for determining a swash plate inclination corresponding to the specified operating speed is first inputted into an actuator, the actuator operating speed executed by the command 5 is detected, and the value obtained by multiplying the detected value by a gain G1 is fed back and added to the first input command value. The added point thus obtained is further multiplied by a gain G2 so as to obtain the inclined position quantity of the swash plate, the swash plate inclined position obtained by the speed command 5 is detected, and the correction value obtained by multiplying the detected value by a gain G3 is fed back and added to the inclined position quantity value of the swash plate. The value obtained by multiplying this added point by a gain G4, a PID control element, is transmitted to a driver 1, a control element, and the operating quantity of a swash plate controlling controller 2 is determined by the operating quantity outputted from the driver 1, thus determining the swash plate inclined position so as to control the discharge of a pump.