PURPOSE: To obtain device which is short in resonator length while it is of external mirror type and yet prevents the peeling of reflecting surface owing to plasma by using the plane mirror with the waveguide having the same diameter as that of a laser waveguide.

CONSTITUTION: A plane mirror 9 and one end of waveguide 8 are in extreme proximity and other ends are cut to a Brewster angle. These are connected to an external mirror laser tube via laser light transmitting window 7. In this case, the waveguide axis a-a' of the laser tube and the waveguide axis b-b' of the reflecting mirror need be deviated by the distance (d) relating to the refractive index (n) and thickness (t) of the Brewster window material. Reducing the optical distance n × t between the waveguide end of the laser tube and the waveguide end of the reflecting mirror as small as possible by selecting (n) low and reduding (t) reduces coupling loss.