PURPOSE: To acurately detect the resistance change of heater wire by holding an electromotive force compensating voltage for heat of the heater wire along with intermittently applying a constant electric power to the heater wire and detecting the change portion of a voltage between both ends of the heater wire with a current made to flow to the heater wire.

CONSTITUTION: The heater wire 2 is embeded in a sample 1 of liquid or powder, etc. In a period when switches s2 and s3 are closed, a voltage equal to one generated between both ends of the heater wire 2 at the condition that the current is interrupted is generated in an integration circuit consisting of an amplifier 14 and a capacitor 15, and held also in the period when the s3 is opened. When the s1 and s2 are closed with this condition, the constant current sent out from a constant current source 12 is made to flow to the heater wire 2, and the voltage generated between both ends of the heater wire 2 is sent out from an output terminal 20 through an operational amplifier consisting of an amplifier 18 and a resistor 19. Consequently, only the voltage applied to the heater wire 2 by the constant current is observed by the terminal 20 without including an erroneous voltage according to a super electric power, etc., gener ated in the heater wire 2, thereby the accurate resistance change can be detected.