PURPOSE: To reduce the closing speed of a diaphragm valve for preventing water hammer due to the rapid closure by employing a swirl chamber at the inlet of a water extracting hole communicating a water flow passage to a back pressure chamber formed with the diaphragm valve.

CONSTITUTION: A swirl chamber 2 is formed at the inlet of a water extraction hole communicating a water flow passage to a back pressure chamber 25 formed with a diaphragm valve 1. Water is made to flow into the swirl chamber 2 from the circumferential direction of the swirl chamber 2 having a water extraction hole opening in the center thereof. The swirl of water in the swirl chamber 2 increases the flow resistance, delays the water supply to the back pressure chamber 25, and necessarily reduces the closing speed of the diaphragm valve 1.