A gas analyzer using a quadrupole mass spectrometric method etc. is provided with an ionizer 211 to ionize a sample gas, a first ion detector 212 and a second ion detector 213 each configured to detect a respective ion from ionizer 211, and each being disposed a respective distance from the ionizer 211 on an opposite side of the ionizer 211, the respective distances being different from each other, a filter 214 interposed between the ionizer 211 and the first ion detector 212 to selectively allow ions from the ionizer 211 to pass therethrough, and an arithmetic device 3 to correct a partial pressure PP.sub.1 of a specific component obtained from the first ion detector 212 and selected by the filter 214 by using a first total pressure TP.sub.1 of the sample gas obtained from the first ion detector 212 and a second total pressure TP.sub.2 of the sample gas obtained from the second ion detector 213.