To prevent the generation of a vacuum part at the time of a lower piston ascending inside a cylinder by repeating opening and shut-off of a hole provided in a boundary face between a fluid storage tank and a lower cylinder by the vertical motion of a cover in response to piston motion in the cylinder.

A fluid storage tank 5 is provided having an air vent 20 at an upper part. A hole 9 is opened in a boundary plate 8, and a connecting rod is provided having an upper cover 6 and a lower cover 7 of larger cross section than the hole 9. At this time, the upper cover 6 is positioned in the storage tank 5, and the lower cover 7 is positioned below the boundary plate 8. With this structure, when a lower piston 3 descends, the lower cover 7 is also lowered, so that a fluid 4 flows into a cylinder from the storage tank 5 and fills the upper part of the lower piston 3. When the lower piston 3 ascends, the lower cover 7 also ascends being pushed by the fluid 4 so as to be force-fitted to the lower face of the boundary plate 8, thereby cutting off communication between the storage tank 5 and the cylinder. The boundary plate 8 is therefore shut off as long as the fluid 4 is pressed by an upper piston 1 so as to generate no space in the cylinder.