The temperature controller adapts a microprocessor for controlling the proportional fuel valve and air fan. The temperature detector detects the temperature of the outlet water to perform the proportional feedback control with the error between the detected and refered temperatures. When there is the error, the heater controls the heating temperature with the differentiate and integral controls so that the error is quickly compensated without the offset error. The detected analog signal is converted to the digital one by the A/D converter so that the control program is simplified.
