PURPOSE: To remove a noise component without delaying a signal by separating the noise component from the detection voltage of a thermocouple through a high-pass filter, and processing the separated noise component and detection voltage together.

CONSTITUTION: A heater 3 heats up by the application of an alternating current 2 phase-controlled through a triac 1, and the electromotive force of a thermocouple 4 coupled thermally and electrically with the heater 3 is amplified by a differential amplifier 5. The output of the amplifier 5 is passed through a high- pass filter 10 to separate its noise component, which is applied to a differential amplifier 12 through a buffer amplifier 11. The differential amplifier 12 amplifiers the separated noise component and the output of the differential amplifier 5 to output a detection output free of a noise component.