To detect a gas to be detected without using a filter by setting a thickness of an infrared absorption temperaturedetecting film to be an integral multiple of a 1/4 wavelength of infrared rays to be detected.

An infrared absorption temperature-detecting film 7 used as a temperature-detecting film is formed of a thin film of an integral multiple of a 1/4 wavelength of infrared rays to be detected. The infrared rays of the wavelength to be detected are resonated and absorbed in the detecting film 7, whereby a high absorptance is obtained. At the same time, the infrared rays of the wavelength to be detected are selectively absorbed, thus exerting a selection function for the wavelength. The need for separately forming a film for absorbing the infrared rays is eliminated. The entering infrared rays are passed through an upper electrode 5, absorbed in the detecting film 7, reflected at a lower electrode 4, absorbed again in the detecting film 7 and reflected at the upper electrode 5. This multi reflection improves the absorptance and enhances detection efficiency.