PURPOSE: To provide a method and an apparatus capable of easily measuring a frequency modulation characteristic of a laser with a small error even when a modulation amplitude is great and of easily correcting a gas signal.

CONSTITUTION: A driving current of a laser 20 of which an optical intensity and optical frequency are varied in accordance with the driving current and the temperature is modulated. The modulated laser light is made to penetrate through a gas cell 27 that includes an absorption gas of which absorption rate and full width half maximum are known. The intensity of the penetration light is detected and a specific component of the detected signal is detected by a phase-sensitive detection so that a modulation characteristic of the laser 20 is measured from the detected signal.