Abstract: In the case of a gas in which several gases are mixed, a type and concentration of the gas may be incorrectly measured when measured using only an optical band-pass filter. The invention of the present application is directed to providing a technology in which a plurality of broadband band-pass filters having overlapping regions are provided to calculate a magnitude of absorption for each wavelength band for light passing through each broadband band-pass filter, thereby identifying the presence of a gas of interest and the presence of a gas other than the gas of interest.