To attain size reduction and a high image quality by providing a second cylinder, which has an opening on the filter side and is cooled to the almost same temperature as a tip part of a second inner cylinder, on an infrared ray detecting element.

A first inner cylinder 4 is joined to a stand 1 of an end part of an outer cylinder 2 joined to a window 3 to pass infrared rays. A second inner cylinder 5 is joined to the other end of a disk 6 joined to an end part of the first inner cylinder 4, and an infrared ray detecting element 7 is arranged in its tip part. A filter 9 to pass only a selected wave length is joined to an opening part on the window 3 side of a first cylinder 8. A second cylinder 20 having an opening on the filter 9 side is arranged on the infrared ray detecting element 7. The second cylinder 20 is cooled to the almost same temperature as the second inner cylinder 5, and is composed of, for example, titanium alloy. By being constituted in this way, unnecessary infrared rays 21 from the first cylinder 8 are shielded by the second cylinder 20, and do not reach the infrared ray detecting element 7. Therefore, since the first cylinder 8 is enough if only it is cooled to about 130 to 170K, cooling capacity required of a cooling bar of a cooler 13 can be reduced.