Abstract: A device for measuring the thickness of a thin film or the like is provided using infrared rays. A source of infrared rays is positioned on one side of the film. A disk with two apertures is rotatably mounted between the infrared source and the film, with band-pass filters positioned over the apertures in the disk. A concave hemispherical reflector with a reflector surface facing the film is positioned between the infrared source and the film. A hole is formed in the center of the concave reflector through which the infrared rays pass. A convex reflector is positioned between the film and the concave reflector and has a reflecting surface facing the concave reflector. A light collecting guide with a photoelectric sensor is positioned on the opposite side of the film from the infrared source for measuring the intensity of the infrared rays.