A valve (100) includes a body (1) having a valve seat (10) formed therein, a bonnet (2), a diaphragm (3) supporting a valve element (18) abutting against and moving away from the valve seat (10), and an elastic member such as an O-ring (5). The body (1) is formed with an annular groove (13) extending around the valve seat (10), and the diaphragm (3) is provided at the peripheral edge (20) with an annular engagement part (21) formed to have an approximate L-shape in cross-section. The annular engagement part (21) is inserted into the annular groove (13) in a tight state by the elastic member disposed between the bonnet (2) and the annular engagement part (21) of the diaphragm (3).
