In analyzing the concentration of an objective component contained in a sample gas by a gas analyzer, an output of the gas analyzer is corrected at a fixed point by applying the relationship between the concentration of a coexistent gas and a span sensitivity of the objective component to make it possible to simply correct the effect of the coexistent gas. In addition, by inputting, at the time of the calibration of the gas analyzer, the information on to what extent the base gas composition differs from the average amount of the actual sample along with the concentration amount of the objective component in the calibration gas, the sensitivity adjustment coefficient is determined by taking into account the effect amount which is previously stored in the gas analyzer, by which the effect amount of the span sensitivity by the difference of the base gas compositions in the calibration gas and the sample gas is suppressed, and the effect of the coexistent gas is canceled to the best possible extent.