PURPOSE: To obtain a complete sealing performance by loading a soft, elastic seal member suitably thicker at its side to be subjected to high pressure between a valve box and a piston.

CONSTITUTION: A seal member 11 is tightened and fixed between a flange 13 of a valve box 1 and a flange 14 attached to a valve cover 2. When water is fed, the water flows from an inlet passage (a) into a valve casing or a piston chamber (b), and then flows through the valve opening into an outlet passage (c). Because the seal member 11 is sufficiently soft and elastic, and an internal force homogeneously acts in the seal material, slacks of the seal member 11 are pressed by the pressure in the piston chamber (b) to form adequate configuration. Thus, the piston side space is completely separated from the piston chamber (b) by the seal member 11.