An FTIR measurement is conducted on a background gas to obtain a single beam spectrum SB(BG) [C] and a synthetic single beam spectrum SSB(BG)[D], and an FTIR measurement is conducted on a sample gas to obtain a single beam spectrum SB(Samp)[E] and a synthetic single beam spectrum SSB(Samp)[F]. A double synthetic absorbance spectrum DSAbs of the sample gas as expressed by the following formula (Step T9) is calculated to obtain a concentration of a trace component (impurity) contained in the sample gas: DSAbs =−log[SB(Samp) SSB(BG)/SSB(Samp) SB(BG)]