PURPOSE: To permit complete sealing for a main water inlet by forming a main body portion incorporating a sealing body with a sloped surface partially surrounding the central valve seat to prevent a tendency such that the sealing body is distorted by closing pressure.

CONSTITUTION: A sealing body 13 is expanded by pressure of water 10, so that an expansion gap is left behind between the seal 13 and a valve seat 16. Subsequently, by the expansion of the seal member 13, the expansion gap is completely occupied by a sealing material 13 expanded to be superposed like a nest with respect to the valve material 16. When the water level in a float chamber is again lowered, and water again enters the float chamber 1 through a pipe 11, a jumper valve 15 again takes a position to the pipe 11 and the seal 23 comes into contact therewith. Preferably the thickness of the seal 13 is about 2 mm, and preferably the gap 17 is about 1 mm.