PURPOSE: To allow the angle adjustment of a parabolic mirror by an angle adjusting mechanism while observing the reflected light reflected by a reference plane provided to the parabolic mirror and a reflecting mirror with a hole by taking out the He-Ne laser light reflected from the above-mentioned plane and mirror to the outside of an optical path.

CONSTITUTION: The reflecting mirror 12 for the He-Ne laser light is first aligned to the optical axis of CO2 laser light 8. The laser light 9 from an He-Ne laser light oscillator 2 is oscillated. This laser light 9 is so reflected by the reflecting mirror 12 that the optical axis thereof aligns to the optical axis of the laser light 8 oscillated form a CO2 laser oscillator 1, i.e. the optical path 10. The reflected laser light 9 is made incident to the reflecting mirror 16 below the optical axis adjusting mechanism 11 and is reflected upward so as to be reflected by the upper reflecting mirror 17, by which the laser light 9 is parallel moved upward so as to enter the hole 18a of the reflecting mirror 18 with the hole. This incident light 9a is reflected by the optical path reflecting mirror 3 and a processing head reflecting mirror. The reflected light is cast perpendicularly to the reference plane 6b of the parabolic mirror 6 and is reflected by said plane. This reflected light 9b is further reflected downward by the reflecting mirror 18 with the hole. The reflected light 9b is projected as a reflection mode 22 on a screen 21 on the outside of the optical path 10.