PURPOSE: To obtain cheap and workable semi-transparent mirror which has suppressed secondary laser beam output by providing the multi-layer interference reflection film on the surface of transparent substrate of which both sides are inclined or deflected and the absorbing film having the laser beam window on the other surface.

CONSTITUTION: The total reflection mirror 31 having the interference reflection film 33 with reflection coefficient of 100% and the semi-transparent mirror 41 having the semi-transparent interference reflection film 44 of 1∼10% are opposingly provided in both sides of the laser tube 36, and the laser is oscillated 30. Both sides of mirror 41 are inclined by about 10min, the positive resist 42' is exposed and developed and thereby the window 45 of the primary laser output T12 is formed. A size of window 45 is determined in accordance with thickness of substrate 41, inclination angle of surface θ and diameter of beam T12, and thereby the secondary beam T21 by multiple reflections within the semi-transparent mirror substrate 41 can be attenuated without attenuation of primary beam T12. It is specially effective to a simple He-Ne laser and is also effective to a low output Ar laser.