PURPOSE: To optimize combustion by a constitution wherein a fan is controlled by the output of the combustion and the set amount of combustion while a proportional control valve is controlled by the state of combustion and the supplying amount of air respectively in the combustion device of a combustion type water heater provided with the proportional control valve for supplying fuel and the fan.

CONSTITUTION: The output signal of a thermistor 52 and the reference value of a volume 54 for regulating temperature are inputted into a fan control circuit 61 to control the rotating speed of a fan 33 so as to obtain the necessary amount of combustion air. The rotating number of the fan 33 is detected by a detecting circuit 62 to input it into a proportional control circuit 63 and control the opening degree of a proportional control valve 45 based on the output of a thermocouple 53. According to this constitution, a proper combustion may be effected and the generation of poisonous gas may be restricted.