A cooling type infrared rays detecting apparatus is disclosed which prevents a fluctuation of an infrared rays image arising from a variation of the cooling temperature. The infrared rays detecting apparatus includes a dummy element mounted adjacent an infrared rays detecting device on a device mounting portion of an inner tube. The dummy element has a similar structure to that of each of infrared rays detecting elements of the infrared rays detecting device and is formed from the same material as the latter. A temperature variation of the dummy element is detected from a variation in resistance of the dummy element, and an output of the infrared rays detecting device is compensated for in response to the thus detected temperature variation to prevent a possible fluctuation of an infrared rays image arising from a temperature variation. Consequently, an infrared rays image of a high quality can be obtained.