A correction curve (FIG. 19) is prepared by plotting 12CO2 concentrations and 13CO2/12CO2 concentration ratios which are determined on the basis of a calibration curve and 13CO2 and 12CO2 absorbances of gaseous samples having the same 13CO2/12CO2 concentration ratio but known different 12CO2 concentrations. A gaseous test sample containing 13CO2 and 12CO2 as component gases is introduced into a cell, and spectrometrically measured. A 12CO2 concentration of the gaseous test sample is determined by way of the spectrometric measurement. A concentration ratio correction value is obtained on the basis of the correction curve and the 12CO2 concentration of the gaseous test sample thus determined. A measured 13CO2/12CO2 concentration ratio is divided by the concentration ratio correction value thus obtained for correction of the 13CO2/12CO2 concentration ratio. Thus, the measurement accuracy of the concentration ratios of the component gases can be improved.