PURPOSE: To prevent the temperature of the hot water stored in a tank from unexpectedly lowering due to the supply of the hot water by a method wherein a first open and close operation mechaism is automtically operated to close based upon the detection of the null state of the amount of stored hot water.

CONSTITUTION: A mechanism 9 to automatically stop the water supply with the filling- up of the tank 2 with water is provided at a feed water pipe 1. In addition, the solenoid-operated first open and close operation mechanism 10 is annexed to the feed water pipe 1 on the upstream side with respect to the mechanism 9. A manually-operated second open and close operation mechanism 12 is annexed to the hot water supply pipe 8. A device 13 to detect that the amount of stored hot water is zero or nearly zero is arranged near the bottom of the tank 2. A device 14 to detect the state that the water supply is at a standstill is annexed to the water supply stopping mechanism 9. Signals from the detection devices 13 and 14 are inputted to a controller 15, resulting in automatically operate the first open and close operation mechanism 10 to open with the detection of the null state by the detection device 13 and, after the mechanism 10 is operated to open, when the water supply is detected to be at a standstill, the first open and close operation mechanism 10 is automatically brought out of function, resulting in automatically allowing to supply water in the tank 2.