PURPOSE: To make it possible to measure gas components of low concentration in a stable state with small drift, by irradiating alternate right and left compartment rooms of a detector with light having passed measurement cells by changing optical paths over at random intervals of time by an optical-path switch.

CONSTITUTION: On the front face and back face of optical-path switch 27, four circular openings AWD and A'WD' are formed and while openings A and A', and C and C' are linked together via straight through holes E and F, B and B', and D and D' are also linked together via curved through holes G and H respectively. Then, bundles of optical fiber is provided in respective through holes to constitute respective through hole as optical paths. Light beams having passed through comparison cell 23 and measurement cell 24 strike left and right compartment rooms 25a and 25b of detector 25 by turns through the rotation of this optical-path switch 27. Therefore, pressure imbalance between left and right compartment rooms 25a and 25b based upon a difference in light energy between comparison cell 23 and measurement cell 24 is doubled. Thus, gas components of low concentration can be measured in a stable state with small drift.