To reduce noise caused by a valve element which sits on an annular valve seat repeatedly.

A valve chamber 3, which communicates an inflow port 4 with an outflow port 5, is formed in valve casings 1, 2. The annular valve seat 7 having a valve port 6 is provided between the inflow port 4 and the valve chamber 3. The disk-like valve element 9, which opens and closes the valve port 6 by sitting on or separating from the annular valve seat 7, is arranged in the valve chamber 3. A cylinder 8 is provided which slidably guides the outer circumference of the valve element 9 in the axial direction of the valve port 6. Outflow windows 14, which are opened when the valve element 9 separates from the annular valve seat 7 and is displaced in the valve-opening direction by a predetermined amount, are provided in the cylinder 8. The width of the outflow windows 14 is formed so as to be broadened as the displacement of the valve element 9 in the valve-opening direction becomes large, and normally-open windows 15, which communicate inside with outside, are formed at the upper portion of the cylinder 8. The valve element 9 normally sits on the annular valve seat 7 to close the valve port 6. When the pressure of the inflow port 4 becomes higher than a set-up pressure value compared with the pressure of the outflow port 5, the valve element 9 separates from the annular valve seat 7 to open the valve port 6 and the valve element 9 is displaced in the valve-opening direction by the predetermined amount to open the outflow windows 14.