PURPOSE: To reliably position a laser light emitted irrespective of laser oscillation or not by providing an absorber for absorbing carbon dioxide gas laser light for emitting in an exciter on the axis of a resonant mirror outside the mirror and passing the measuring laser light such as an alignment.

CONSTITUTION: A control laser oscillator 6 is provided at the position coaxial with the axis 5 at the control laser light L2 emitted from the oscillator outside a water tank 17. Thus, carbon dioxide gas laser light L1 emitted from a carbon dioxide gas laser oscillator which has an alignment and a resonator mirror even during laser oscillation can be positioned. In other words, the light L1 passed through a passing hole 2 during laser oscillation in the alignment passes a transmission window 4, but is absorbed to the water 16 filled in the tank 17 and not emitted out. Similarly, the light L1 emitted from the resonator mirror passes the side wall 20 of a water tank 19, but is absorbed to the water 21 in the tank 19. Thus, even if the resonator mirror is thermally drifted, the control laser light of He-Ne can be propagated always coaxially with the carbon dioxide gas laser light thus displaced.