To close a channel by accurate action even though a foreign matter enteres so as to avoid occurrence of a water hammer phenomenon, by equipping a valve element and a piston to a valve main body having a valve chamber, assisting water pressure resistance against the valve element by the action of the piston, and decelerating the ascending/descending of a float so as to transmit to the valve element and the piston.

When water in a reservoir 1 is discharged from an outflow pipe 3, a float and a vertically moving rod 28 are started to be lowered, and a second pinion engaged with a second rack arranged to the vertically moving rod 28 is started to be rotated. This rotation is transmitted to a first rack and an operating rod 22 through a shaft by decelerating the rotation of a first pinion at a ratio of one third so that the operating rod 22 is started to be slowly lowered. Further, a valve rod is lowered, and the piston is lowered to an upper interior 11a so that water in the reservoir 1 is introduced into the upper interior 11a. A valve element 14 arranged around the valve rod is also detached from a valve seat 13, and the opening 13 of a lower interior 11b is sequentially opened so that water is introduced from an inflow pipe. The increasing speed of the valve element 14 is decelerated against the increasing speed of the float, so that water hammer pressure can be diminished.