PURPOSE: To improve the efficiency of condensation of infrared rays and to make it possible to set an area of detection in proximity, by forming a condensing means of a plurality of concave mirrors whose optical axes are diverged in the direction of condensation and whose focuses are made to coincide together, and of a small-sized concave mirror which is provided in the vicinity of the focuses of said concave mirrors in such a manner that the reflecting surface thereof is opposite to those of these mirrors.

CONSTITUTION: In the vicinity of the focus of a main reflecting mirror 1 composed of a plurality of concave mirrors 1a and 1b and having an infrared ray detector 2 disposed in the central part, a subordinate reflecting mirror 3 formed of a small-sized concave mirror and provided in such a manner that the reflecting surface thereof is opposite to that of the main reflecting mirror 1 is disposed to form a condensing means together with this mirror 1. Since the subordinate reflecting mirror 3 is disposed between incident infrared rays A and B each being parallel to an optical axis of light coming from an area of condensation by each of the concave mirrors 1a and 1b, e.g. from each area of detection, the fall of the efficiency of condensation due to the shadow of the subordinate reflecting mirror 3 is prevented. Moreover, since the subordinate reflecting mirror 3 is disposed in the vicinity of the focus of the main reflecting mirror 1, the angular aperture of the axes of the concave mirrors 1a and 1b of the main reflecting mirror can be made small, and thus the area of detection can be set proximity.