PURPOSE: To achieve alignment even with operation for selecting oscillation wavelength by slightly rotating etalon by allowing an air space etalon to form one end face of an optical member on a spherical reflection surface and to support it so that it can rotate freely with a curvature center of this spherical reflection surface as a support.

CONSTITUTION: An etalon device has an air space etalon 30 which is constituted by two light-transmission bodies 29a and 29b which are connected by allowing reflection surfaces 26 and 27 to be in parallel through a plurality of spacers 25 which are placed at equal interval and by forming an air gap 28 and a supporting means for retaining it. Then, when selecting an oscillation wavelength, for example, one adjusting screw 40a is warmed up for forming a specified gap at a tip part and then the other adjusting screw 40b is advanced by that gap, thus enabling an angle where the reflection surfaces 26 and 27 with band-narrowing and wavelength selection operation and a laser light axis L cross to be changed. On the other hand, since a reflection surface 31 is one part of a spherical surface 33, the air space etalon 30 is supported so that it rotates with a curvature of the spherical surface 33 as a center even when there is a variation due to movement of the adjusting screws 40a, 40b, thus preventing a resonant light axis relating to an output mirror 32 from being changed at all.