To easily collectively determine a plurality of components constituting a compound regardless of the degree of similarity between the shapes of absorption spectra by assuming part of a plurality of components as the spectrum of a single component and using the assumed spectrum as a reference spectrum in preparing a calibration matrix for computing concentration.

A resultant spectrum 13 is assumed by synthesizing, for example, the absorption spectrum 11 of formic acid (HCOOH) and that 12 of acetic acid (CH3COOH) which belongs to the same carboxylic acid (R-COOH) as that of the formic acid on a computer. In the case of preparing a calibration matrix for computing concentration by changing the concentrations of the acids in six stages of, for example, 0, 20, 40, 60, 80, and 100, the total quantity of the mixture of the acids can be collectively calculated as one compound group when the spectrum 13 is used as a reference spectrum.