PURPOSE: To impact an appropriate temperature range and a central temperature to an object to be picked up of image to optically scanning the object to be picked up, and displaying the object to be picked up as a substantial central temperature of Tmax-(1/2)TW or Tmin+(1/2)TW with respect to the difference TW between maximum temperature signal Tmax and the minimum temperature signal Tmin.

CONSTITUTION: A radiation from each part of an object 1 to be picked up is converted into a temperature signal through an optically scanning mechanism 2, an infrared ray detector 3 and a correction circuit 5. Its output signal is supplied to a signal converting circuit 8 or an IC memory 9 via an A/D converter 6 and a switching circuit 7. The output signal from the memory 9 is supplied to an operation control part 10 through a synchronizing signal generator 14, and a temperature width TW obtained by dividing n(2 through 5) equal parts of the difference between the maximum temperature signal Tmax and the minimum temperature signal Tmin. The output signal enters a converting circuit 8 and the central temperature is converted into Tmax-(1/2)Tw or Tmin-(1/2)TW, and the object to be picked up is displayed of its illumination with an appropriate temperature width and the central temperature on a cathoderay tube 13 via a D/A converter 11 and an amplifier 12.