PURPOSE: To improve reliability and EMI(electromagnetic interference)-resistance of an actuator by providing a controller which inputs the output of a position detector of a control piston and an actuator as a feedback input and sends a control signal to a servo valve by a position command signal.

CONSTITUTION: A variable swash plate type hydraulic pump 4 is driven by an AC motor 3 in a specified direction at a specified speed. When a position command signal 2s is input into a controller 2, the controller 2 receives this signal and feedback signals from a position detector 5a, 5 of a control piston 14 and an actuator 6 to send a control signal responsive to the position command signal 2s to a servo valve 8. The servo valve 8 operates a variable swash plate via the control piston 14 in response to the input. Accordingly, the variable swash plate type hydraulic pump 4 changes discharging amount of flow and direction in response to the control signal to operate the actuator 6. The controller 2 indirectly controls the hydraulic pump 4 via a servo valve 8 having small inertia without controlling the AC motor 3.