PURPOSE: To eliminate the loss of optical energy caused at the convergence of light without increasing a light source power by periodically moving a sample cell and standard cell arranged in parallel to each other integrally.

CONSTITUTION: A sample gas is passed in a sample cell 1, and nitrogen gas is sealed in a standard cell 2. The cells 1, 2 are arranged in parallel to each other and fixed to a moving base 5. A light source 3 and a detector 4 are set on the same line in the state where they are opposed to each other with a distance capable of inserting each cell between them. The cells 1, 2 are alternately moved between the light source 3 and the detector 4 by the drive of a motor 8. When the cell 1 is present between the light source 3 and the detector 4, the optical absorption of the sample gas is measured. When the cell 2 is present between them, the stable measurement of the sample gas concentration by detecting a standard signal, and feeding back the standard signal by the continuous operation of the movement is continuously carried out. All the emitted optical energy from the light source 3 is projected to the cell 1 or 2, and the light passed through the cell is introduced to the detector 4 without any loss.