PURPOSE: To maintain temp. of hot water supply at a constant value by inputting outputted signals from bridge circuit consisting of temp. sensing element, variable resistor, fixed resistor and the like into the drive part of the proportional control valve intermittently.

CONSTITUTION: When the bridge circuit at balanced status loses its balance by lowering temp. of hot water supply caused by an increment of cold water being poured into the hot water, it increases the resistance at the thermistor 1 which generates potential difference. The generated potential difference is inputted into the amplifier 5, and the amplified output actuates the drive part 6 of the proportional control valve 7 only during the period when the contact 9a of the relay 9 which is made at ON status by the actuation of the oscillator 10 is closed. After the elapse of a prescribed period of time, oscillator 10 intercepts the operation of the drive part 6 by making the relay 9 at OFF status, and secures the delayed time lengths (a) until the moment temp. of hot water supply reaches the value which corresponds to the change of input at burner 8 caused by the actuation of the drive part 6, and (b) until the moment the thermister 1 detects its temp. After the elapse of the above mentioned time length, the same movement is repeated when occasion demands, and the temp. of hot water supply is made stabled at the originally prescribed value.