To obtain a gas sensor which can be miniaturized and simplified by a method wherein infrared light radiated from a light source is reflected by a concave reflecting mirror and its reflected luminous flux is converged separately by two elements whose sensitivity wavelength is different.

A circuit board 4 and a cancave reflecting mirror 5 are housed inside a metal case. In addition, a light source 6 and a photodetector are arranged on the circuit board 4. A filter holder 8 is arranged on the circuit board 4 in a state that it covers the photodetector. Two light-receiving parts whose sensitivity wavelength is different are built in the photodetector. That is to say, the light-receiving part on one side is constituted of a dual element which is provided with an electrode for temperature compensation and with a light-receiving electrode for CO2, and the light-receiving part on the other side is constituted of a dual element which is provided with an electrode for temperature compensation and with a light- receiving electrode for comparison. A reflected luminous flux which is reflected by the reflecting-mirror face (a) of the concave reflecting mirror 5 out of infrared light radiated from the light source 6 is converted on the light-receiving electrode for CO2, and a reflected luminous flux which is reflected by its reflecting-mirror face (b) is converged on the light- receiving electrode for comparison. Since an optical path R and an opticap path S are formed separately, the condensing efficiency of a gas sensor is good.