To prevent the distortion of signals accompanying the saturation of an amplifier generated in the case that a large difference is present between the luminance level of an optical system inner surface and the luminance level within a view field with respect to a scanning type infrared ray image pickup device provided with an optical scanning mechanism such as a rotary polygon mirror and a vibration mirror, etc.

The output of an infrared ray detector 4 is distributed into two systems and a first system 14 performs output to a signal switching device 16 as it is without working. A second system 15 outputs average output obtained from the infrared ray detector 4 in a valid scanning area to the signal switching device 16. The signal switching device 16 is controlled by switching signals 20, selects the signals of the first system 14 in the valid scanning area and the signals of the second system 15 in an invalid scanning area and outputs them to a post-amplifier 8.