PURPOSE: To supply hot water stably and comfortably at all times by constituting a device in such a manner that the opening of a proportional control valve is controlled in response to the difference between the detected temp. and set temp. of hot water and the detected water flow rate flowing through a heat exchanger.

CONSTITUTION: When a hot water faucet 3 is opened, water flows through a water pipe 1, which is detected by a control part 10. The part 10 opens a proportional control valve 7 after detecting the water flow and operates an ignitor. A heat exchanger 2 is heated by the ignition of a gas burner 5, and the water flowing through the pipe 1 becomes hot water and is discharged from the faucet 3. At the time of hot water supply, the part 10 detects the hot water temp. flowing through the pipe 1 with a temp. sensor 4 at every fixed time. The combustion quantity is not only controlled by the control difference between the detected temp. and the set temp. previously settled at an operation part 11, but also the control of the combustion quantity is corrected by the detection of the volume of the water quantity with a water quantity sensor 20. Therefore, an optimum combustion quantity corresponding to a hot water supply condition is quickly obtained to make the undershoot or overshoot of the hot water temp. small.