PURPOSE:To reserve water of a proper temperature continuously and automatically by a method wherein mixing control is carried out by detecting temperatures for both hot water and cold water. CONSTITUTION:A temperature sensor 8 and a solenoid valve 6 are provided on a hot water delivery pipe 5 to be led from a heating tank 1, a temperature sensor 9 and a solenoid valve 3 are provided on a water pipe diverged into two ways, outputs of the temperature sensors 8 and 9 are applied to a control circuit 10, an input is applied to an operational amplifier A1 and A2 for level comparison, where an output of a serrated pulse oscillation circuit PUL is to be applied as a reference value, openings of the solenoid valves 3 and 6 are adjusted, and mixture quantities in a hot water reservoir 4 are controlled. 7 is a water level detecting switch.