PURPOSE: To detect water leakage at its early stage by detecting water discharge by means of the opening and closing of a valve body and judging water leakage when the water discharge continues for a specified period.

CONSTITUTION: When an electro-magnetic mechanism 28 is closed and a water supply cock 3 is closed, the pressure on the side of a primary chamber 12 that is introduced into a pressure chamber 19 does not escape to the side of a secondary chamber 13, and a pressure receiving body 17 is pushed up and a valve body 15 is closed. When the water supply cock 3 is opened or water leakage occurs, the pressure in the pressure chamber 19 escapes through pressure release channels 26 and 27, and the pressure receiving body 17 is pulled down, opening the valve body 15 to supply water. The opening and closing of the valve body 15 is detected by a microswitch 24 and it is sent to a control circuit 5. When the valve body is opened for a time longer than a specified time, it is judged that closing of the water supply cock is forgotten or there is water leakage in the running water pipe 1, and an alarm device is operated.