PURPOSE: To compensate interference owing to a coexisting gas by subtracting the output of a sensor which responds to the concn. of the interfering gaseous component in a sample gas from the output of the concn. of the component in the gas to be measured in the sample gas and the concn. of the component in the coexisting gas which coexists in the sample gas.

CONSTITUTION: The measuring luminous flux M from a luminous flux distributor 2 and a reference luminous flux S are intermitted alternately by a chopper 3 and are made incident via an interference cell 4 to a measuring cell 5 and a reference cell 6. There is no IR absorption in the cell 6 and the flux S is made incident to the chamber 7B of a detector 7. The flux M transmitted through the cell 5 absorbs a part of IR rays by a sample gas SO2 and H2O and is made incident to the chamber 7A of the detector 7. The output from the detector 7 is the sum of the outputs for the gas SO2 to be measured and coexisting gas H2O in the sample gas and since the output from a DA converter 18 is the output for the coexisting gas H2O, the differential output from a differential amplifier 19 is only the gas to be measured SO2 and the interference by the coexisting gas is compensated.