To prevent the reciprocation of a piston rod caused by its forward displacement and to prevent the generation of the excess stress on a front spring by expanding only a rear spring chamber during the work.

A rear spring chamber R3 and a front spring chamber R4 can communicate with a front cylinder chamber R1 or a rear cylinder chamber R2 through an exhaust passage 44 by forming a rear spring chamber vent hole J3 and a front spring vent hole J4. Particularly by communicating the rear spring chamber R3 and the front spring chamber R4 by the exhaust passage 44, the rear spring chamber R3 and the front spring chamber R4 can be constantly kept at the same atmospheric pressure. Accordingly only the rear spring chamber R3 is kept in a state of high atmospheric pressure, which prevents the forward displacement of the reciprocation of a piston rod 28, and prevents the deterioration and the failure of the front spring 39.