PURPOSE: To prevent the deflection at the middle of supporting members and thereby to prevent the dislocation of an laser resonator optical aixs by a method wherein the middle portions of supporting members mounting optical system components and made of an alloy of a low expansion factor, are fixed to vibration-proof rubber and the two ends of the supporting members are supported by the vibration-proof rubber movably along the length of the vibration-proof rubber.

CONSTITUTION: Supporting members 5 are constituted for example of invar, whose thermal expansion factor is only one tenth that of an ordinary iron-based material, which moderates the thermal impact due to electric discharge produced by a laser in operation. The middle portions of a lower pair of the supporting members 5 are fixed by screwing frames 10 to mounts 9 by means of screws 11, and the mounts 9 are fixed to the mounts 9 with the intermediary of vibration-proof rubber 7. The right and left ends of the supporting members 5 are movable along the length of mounting sections 8 on the vibration-proof rubber 7. Changes in the dimensions of a resonator due to temperature changes within an oscillator 1, or the changes in the length of the supporting members 5 made of invar or the like, are absorbed by the ends of the supporting members that move freely. Thermal stability is ensured for an optical system, different from the conventional design wherein only one-sided change takes place, because the thermal distortion is divided to be borne by the respective ends of the supporting members 5.