PURPOSE: To remarkably reduce the cost of the device in the caption by providing a C-shaped wire spring symmetrically between spring engaging parts on both side parts of a piston and a valve working body of the center portion thereof.

CONSTITUTION: Spring engaging parts 41 are integratedly formed on both sides of a piston 21. C-shaped wire spring 43 is provided between recessed grooves 42 of the spring engaging parts 41 and a valve working body 46. As a result, number of parts can be remarkably reduced in comparison with a conventional device using compressed coil springs. In addition, the cost is considerably reduced because the installation of a wire spring is extremely facilitated.