PURPOSE: To reduce a radio frequency inductive trouble or a flicker by a method wherein both of an ON-OFF control and a phase trigger control, triggered by a limited phase, are employed.

CONSTITUTION: A heater 14 is controlled by a method wherein a semiconductor element 15 is put ON or OFF in an ON-OFF circuit 20 by a signal from a reserved hot- water temperature sensor 18 while the semiconductor element 15 is triggered by a phase trigger control circuit 30 at the limited phase. The temperature of water is low at rising up, therefore, a comparator 25 is put OFF, a transistor 13 is put OFF, the transistor 30 is put ON and the semiconductor element 15 is triggered through a pulse transformer 17, thereby conducting the heater 14 fully. When the temperature has approached a set temperature, the comparator 25 is put ON, the transistor 30 is put OFF to stop the ON at the zero closs thereof, and the semiconductor element 15 is triggered at the phase, which is constant at all times, to reduce the heating value of the heater 14. When the temperature thereof is arrived at the set temperature, the power supply of the heater 14 is stopped and heating or stopping is repeated to keep the temperature under a condition that the heating value of the heater 14 is reduced.