PURPOSE: To enhance measuring accuracy by improving range correlation between a plurality of measuring ranges.

CONSTITUTION: A gas analyzer comprises a concentration detector 10 for outputting a gas concentration signal, a variable range type multiplier 12 for multiplying the signal by a magnification corresponding to range data to output it, and a calculator 15. The calculator 15 outputs range data of a selected measuring range to the multiplier 12, and calculates gas concentration based on calibration curve data of the range selected from the output of the multiplier 12. The calibration curve data of the measuring range except a maximum measuring range stored in the calculator 15 is obtained by using the range data of a maximum measuring range and the range data obtained from the calibration curve data, and the range data of the respective measuring range except the maximum range is obtained by correcting the range data of the maximum measuring range and the range data from the calibration curve data by using span gas of the respective measuring ranges.