PURPOSE: To provide the constitution of a fire sensor for preventing a filter plate from coming off in a reception window part and easily assembling the reception window part.

CONSTITUTION: The reception window part 3 is constituted of a photodetector for receiving infrared ray wireless signals, a containing frame 3f for containing the photodetector and the filter plate 3e fitted near an opening 3a provided on the containing frame 3f for shielding the diffused external light of sunlight and a room lamp, etc. Also, in the reception window part 3, filter supporting means 6 and 7 abutted to the surface of the filter plate 3e are provided near the opening 3a of the containing frame 3f and four catches 5 engaged to the back surface of the filter plate 3e are provided on the internal wall of the containing frame 3f.