To fix an optical element for a laser with which pressing force is uniform, mounting is easy and the efficient cooling of the optical element is possible by supporting the one surface of the optical element by means of a brush consisting of thermally conductive fibers and fixing the optical element to an optical element supporting member.

The optical element supporting member 5 is provided with a blind optical element mounting hole 7 mounted with a total reflection mirror 3. The thermally conductive fibers 11 having a diameter of, for example, 5 to 100 μm are densely planted and adhered by a thermally conductive adhesive 13 to the bottom 9 of the optical element mounting hole 7. Metallic fibers consisting of, for example, Fe, Al or Cu, etc., their alloys or carbon fibers of a PAN system or pitch system are adequately used for such thermally conductive fibers 11. Methods, such as brazing or 'caulking' or press fitting in addition to the adhesives are usable as means for implanting the thermally conductive fibers 11 to the optical element supporting member 5 at the time of using the metal fibers for the thermally conductive fibers.