To compensate fluid pressure losses in a closed system and fill a closed system by an ordinary filling device.

A piston 1 of a working cylinder slides in a casing 2 to divide the casing into two working chambers 3, 4 containing a hydraulic fluid, one of the working chambers 3 capable of being energized by a hydraulic control device 6 through a control line 5. The hydraulic chamber 4 forming a closed system includes a fluid connection part communicating with a valve. The valve connects the hydraulic chamber 4 to a filling device 8 during a filling operation and after the termination of the filling operation, the valve connects the hydraulic chamber 4 to the hydraulic control device 6 for the purpose of compensating fluid losses. By the valve 7, at the first filling time, the hydraulic chamber 4 is connected to the filling device, thereby performing the combined load/ pressure filling process. At the start of operating the working cylinder, the valve is automatically switched so as to compensate fluid losses through the hydraulic control device 6.