PURPOSE: To provide a hydraulic actuator having a simple structure and is easily assembled by utilizing a space for containing a piston rod in an actuating cylinder as a pressurized fluid reservoir, and by equipping the inlet/outlet control of the pressurized fluid through arranging a directional control valve between a pump and the actuating cylinder.

CONSTITUTION: A structural unit 1 comprise an actuating cylinder 2 including an actuating piton 3, a pump 5 driven by a motor 6, a directional control valve 8, and a reservoir. A space 13, which receives an actuating piston rod 4 in the actuating cylinder 2, is utilized as a storage reservoir for the working fluid. In the output stroke of the actuating piston 3, a directional control valve 8 directly connects the space 13 with the intake port of the pump 5. At the end of the output stroke, the directional control valve 8 directly connects the intake port of the pump 5 with its delivery port, and also lowers the pump pressure by means of a switching means. Furthermore, an acting side A1 of the actuating piton 3 is connected to the space 13 to return the piston 3 to its initial position.