PURPOSE: To improve the response sensitivity and measurement accuracy of an infrared sensor by preventing the influence of the Joule's heat of two temperature-sensitive sections on the output of the sensor which is the difference between the self-generating quantities of heat of the two sections.

CONSTITUTION: A first infrared-ray detecting means 11 detects the quantity of infrared rays radiated from an object 13 to be measured for temperature and second infrared-ray detecting means 12 detects the infrared rays radiated from an infrared-ray radiating source 14. The output signals of the means 11 and 12 are sent to a difference detecting means 15 and the means 15 detects the difference between the two output signals. The output signal of the means 15 is fed back to an infrared-ray radiating source control means 16. The means 16 controls the output of the source 14 so that the output of the means 15 can become zero, namely, the outputs of the first and second infrared-ray detecting means 11 and 12 can become equal to each other. The current value of the source 14, namely, the value converted into the temperature of the object 13 when the output of the means 15 becomes zero is displayed by means of a displaying means 17.