PURPOSE: To remove stray light, background, heat, noise, or the like due to bachlight and background light, and to reduce the rising time of a detector by superimposedly fitting a plurality of aperture elements consisting of a metallic thin plate.

CONSTITUTION: A cold aperture 10 is made up of tubular first to third aperture elements 11 to 13 consisting of copper or a metallic thin plate of radio-wave absorptivity and having circular windows 11A to 13A in the central parts of the respective ceiling plates. The element 11 is fixed to the bottom plate 2 of a cylinder 3, and the element 12 is fitted to the element 11 so that the respective ceiling plates have an interval therebetween and the respective outer walls are superimposed. The element 13 is fitted to the element 12 in the same manner. In this case, the shapes and dimensions of the respective circular windows 11A to 13A are successively enlarged so that the visual field angle α of the infrared detecting element 1 has a prescribed value. The axial centers thereof are brought into line with that of the light-receiving surface of the element 1. As a result, stray light and background light hardly reach the element 1, so that background, heat, and noise can be removed. Further, since the aperture 10 is made of a metallic plate and small in heat capacity, the rising time of the detector can be shortened.