PURPOSE: To always obtain the laser output light of a single mode by composing mirrors of several different types having different curvatures for output mirrors when providing the output mirrors and fully-reflecting mirror at both ends of a resonator which contains Ar gas laser tube, monitoring the lateral mode of the laser light and selecting the mirror by the signal.

CONSTITUTION: An Ar gas laser tube 1 composed of fine tubes 5 is contained in a resonator 4 which has output mirrors 2 and fully-reflecting mirrors 3. In this construction, the mirrors 3 are composed of mirrors (a)∼(f) of several different types having different curvatures, held by rotatable mirror holder 10, and first outputted by the mirror (a). In other words, the laser output light 6 from the mirror (a) is split via a beam splitter 7 into two beams 8, 8', the beam 8 is incident to a monitoring scanning interferrometer 9, a mirror holder rotating unit 11 is operated by the mode of the received output light, thereby selecting the optimum mirror.