The inside of a cylinder is divided into front and rear cylinder chambers, a primary-side piston and a secondary-side piston, each formed by an annular disc part and an annular tube part are fitted into each cylinder chamber, with the annular tube parts of each loosely fitted together, and the secondary-side piston surrounding an operating piston rod so as to hold the operating piston and being fit into the operating piston. The first front cylinder chamber and the second rear cylinder chamber defined by the primary- and secondary-side pistons are connected by a communicating passageway with a relief valve inserted therein, this relief valve being switched to the open condition by either a prescribed pressure or a valve opening/closing bar that is provided upright on the primary-side piston.