PURPOSE: To properly detect a decrease in the water temperature due to make-up water and the completion of heating-up of water for properly controlling the water temperature by a method wherein a temperature sensor is installed in a water path of a supply water guide device and at the same level as or higher than that of a heater, and a controller is provided to control a heat source of the heater according to the detected temperature.

CONSTITUTION: Make-up water is led to a water chamber 16 through the inside of a supply water guide device 9. As the supply water guide device 9 is located apart from a hot-water faucet 29, the water is barely mixed with the hot-water even when the water is supplied while hot-water is delivered, so that the hot-water in the upper part of the water chamber 16 can be taken out without much decrease in the hot-water temperature. As a temperature sensor 25 is installed in a water path right below the supply water guide device 9, a decrease in hot-water temperature due to make-up water is quickly detected and a combustion device 18 is brought in operation by a controller 23 while water is supplied, so that the hot-water having a specified temperature can be continuously delivered while water is supplied. Since the temperature sensor 25 is located above a heat exchanger 12 of heater 10 and at nearly the same level as that of a top board of a combustion chamber 11, the hot-water temperature in the upper part of the water chamber 16 can be properly detected and the hot-water is prevented from being heated over the specified temperature.