A valve seat portion 20 has a nearly tubular shape, at the bottom of which a circular opening portion 23 which functions as a valve seat is formed. A valve portion 10 has a ring-shaped supporting portion 11 which is disposed inside the valve seat portion 20. A valve body 12 has a shape corresponding to the circular opening portion 23, and multiple coupling portions couple the supporting portion 11 and the valve body 12. In the valve portion 10, the valve body can move between a closed position in which the valve body closes the opening portion 23 in the valve seat portion 20 and an open position in which the valve body opens the opening portion 23 by the flexibility of the four coupling portions 13.