A volute chamber having an inflow port is provided at the inlet of an extraction hole of a diaphragm valve. The volute chamber is formed with a lid in a hollow portion of a synthetic resin material member of the diaphragm member. The volute chamber is formed with a cap as volute chamber forming piece at the lower portion of the synthetic resin material member. A small projection is formed at the inlet of the extraction hole and protrudes in the volute chamber. A rib having an inflow port is formed partially at the outer surface of the cap. The spinning speed of the water in the volute chamber and the fluid circulation resistance in the volute chamber become large. The water supply to a back pressure chamber reduces and the closing speed of the diaphragm valve decreases. The water hammering action in the water inlet valve can be mitigated due to the rapid closing operation of the diaphragm valve.