PURPOSE: To obtain hot water having desired temperature at all times by mounting a water-quantity regulator interlocked with hot-water temperature setting operation and a water-temperature sensing valve controlling the quantity of flowing water in response to the temperature of entering water.

CONSTITUTION: A proportional solenoid valve 4 fitted to a gas supply path 3 to a burner 1 is controlled by comparing the resistance of the thermo-sensitive resistance element 6 of a hot-water discharge path 5 and resistance value set by means of a hot-water temperature setting dial 7. A flowing-water quantity limit mechanism 9 consisting of the water-quantity regulator 13 turned by means of a servomotor 12 interlocked with the operation of the dial 7 and the water- temperature sensing valve 17 in which a valve body 15 adjusts the opening of a valve hole 16 through the expansion and contraction of a wax thermostat 14 is mounted.