PURPOSE: To improve the operational convenience for a user by a method wherein the residual amount of hot water is displayed steplessly and continuously on a digital indicator.

CONSTITUTION: When hot water is supplied to usage, the water flows in a feed water piping 21, resulting in generating the pulse voltages, a number of which corresponds to the amount of the flow of the water, as the output of a flow sensor 26. Said pulse voltages are sensed by a flow detection circuit 32 in order to detect the amount of the flow of the water. A residual hot water arithmetic circuit 33 calculates the residual amount of hot water by subtracting said amount of the flow of the water from the volume of a boiler body. The resultant signal is transmitted to the indicator circuit 36 of an indicator 27 so as to output on a digital display 28 after being decoded. Further, the residual hot water arithmetic circuit 33 is resent by a heating-up signal, which acts to stop the energizing of a heating device 23 when the water is heated-up by the energizing of the heating device 23. Because the residual amount of hot water is displayed steplessly and continuously on the digital indicator, the residual amount of hot water can be known accurately.