To prevent the leakage of water even when the water suddenly flows through a flow-in port into a valve chest.

A casing consists of a body 1 and a cover 2 to form the valve chest 3 which has the flow-in port 4 opened in the lower part and a flow-out port 5 opened in the upper part. A valve seat 6 is formed between the valve chest 3 and the flow-out port 5. The body 1 has a plurality of ribs 9 integrated with the inner wall of the valve chest 3 and protruded inside, and a float support 10 is arranged inside the ribs 9 where a bottomed and approximately cylindrical through-hole 13 having a large opening area is provided for communicating the inside with the outside thereof. A spherical float 12 is arranged in the float support 10 in a free condition. A water flow receiving member 14 has a normally opened communication hole 15 having a small opening area for opening/closing the through-hole 13. The water flow receiving member 14 opens the through-hole 13 with the energy of water suddenly flowing through the flow-in port 4 into the valve chest 3.