PURPOSE: To eliminate the influence of the optical path length fluctuation of a laser resonator by splitting a beam of light emitted from the laser resonator to be measured, and adjusting the optical path length so that the intensity of interference light resulting from the recombining of the split light beams is held constant.

CONSTITUTION: A beam of light from a laser resonator 101 to be measured is split by a beam splitter 102 into two light beams whose directions of propagation are set to be parallel to each other by a reflecting mirror 103. One of the two light beams is incident on a measuring Michelson interferometer and split into two light beams by a cube beam splitter 104; one of these light beams is reflected by a fine adjustment mirror 105 and the other by a movable mirror 106, and they are recombined at the splitter 104. The recombined light beams are incident on a photodetector 107 and measured. The other light beam from the splitter 102 is incident on a correcting Michelson interferometer and split into two light beams by the splitter 104. For the intensity of interference light resulting from the recombining of the two light beams to be held constant, the optical path length is adjusted.