PURPOSE: To make possible the miniaturization and simplification of a device and the reduction of a measurement time by using a laser beam from a semiconductor laser as excitation light of a vibrational level of a gas molecule or a gas radical at the time when the absorption of the gas molecule or the gas radical is obtained.

CONSTITUTION: A gas to be measured is enclosed in a cell 22 and windows 22a are mounted to face each other. The half part of the laser beam of an AlGaAs semiconductor laser 21 passes the cell 22 by the use of a mirror 23 to measure optical strength by the use of an optical detector 24 and the optical strength of the other half part thereof is directly measured by the use of another optical detector 25. And the former measurement values A and the latter measurement values B are processed by the use of a data processing device 26 and recorded in a chart recorder 27. The oscillation wave length of the laser 21 is changed by changing the temperature with the use of a temperature controller 28, so the oscillation absorption spectrum of the gas of the cell 22 is obtained. A device is miniaturized and simplified and the reduction of a measurement time is made possible.