PURPOSE: To display the image of temperature fluctuation on the surface of a body to be checked accurately, by utilizing the operation control part of a microcomputer and the like, obtaining the extreme value for each scanning line, and displaying only the fluctuating part of the temperature.

CONSTITUTION: An electric signal having the magnitude corresponding to the amount of infrared rays irradiated from the body to be checked 1 is converted into a digital value through an analog to digital converter 5. Thereafter, it is inputted into an operation control part 6 comprising microcomputer 8 and the like. The extreme value is obtained for every scanning line in said operation control part 6. A threshold value is computed for every temperature fluctuating part by said extreme values. Based on said threshold value, the image signal for displaying the temperature fluctuation is outputted to a display part 12 comprising CRT through an interface 11. In this way, the image of the delicate temperature fluctuation on the suface of the body to be checked can be accurately displayed.