PURPOSE: To prevent contamination due to adherence of dusts, fine particles, gas molecules and to increase the life of a resonator mirror by spraying laser medium gas to the surface of the mirror in parallel with the surface of the mirror in a laser oscillator having the mirror, and then sucking the gas to a suction port.

CONSTITUTION: Several sets of opposed diffusers 31 and suction ports 33 are so opened at a mirror holder 30 for fixedly holding a resonator mirror 5 at a predetermined distance from the mirror 5 that a gas stream from the diffuser 31 to the port 33 becomes parallel to the surface of the mirror 5. The diffusers 31 are gathered to one conduit 32, connected to a filter 23, and the ports 33 are gathered to another conduit 32, and connected to a blower 34. Laser medium gas GF containing dusts sucked from the ports 33 through the conduit 32 is fed to a filter 23 through the conduit 32 to remove the dusts, purified gas GFB is diffused from the diffuser 31, and again sucked from the ports 33.