PURPOSE: To improve temperature characteristic upon general use hot-water supplying and prevent excess temperature rise upon high-temperature hot-water supplying by a method wherein a proportional control constant is switched automatically and reversely from a large value to a small value based on the switching from the general use hot-water supplying to the high-temperature hot-water supplying.

CONSTITUTION: A fuel gas supplying path 16 for the heating burner 15 of a heat exchanger 1 is interposed with a solenoid gas valve 17, a gas burner 18 and a proportional valve 19. In case a flame rod 21 detects the flame of gas after the igniting operation of the burner 15, the proportional control constant is increased when a changeover switch 28 is positioned at the general use hot-water supplying position while the same constant is decreased when the changeover switch 28 is not positioned at the general use hot-water supplying position and, thus, the control, in which the proportional valve 19 is operated based on the detection of a thermistor 8 detecting outlet hot-water temperature, is continued.