PURPOSE: To support a Fabry-Perot etalon(FP) without producing a distortion and prevent contamination due to degasification and the like by supporting only one side glass of the FP with U-shaped rings made of fluororesin.

CONSTITUTION: An FP 7 for tuning wavelengths is disposed between a laser oscillator and a partial reflecting mirror. A closed vessel 8 where the FP 7 is housed is filled with a gas and is sealed with anti-reflection(AR) coating glass 9. Only one of glass materials out of two sheets of them which form the FP 7 is supported in the closed vessel 8 by means of U-shaped rings 27 made of a fluororesin. In other words, if two sheets of glass are supported simultaneously, surplus force equal to slippage acts upon and gaps between two sheets of glass produce distortions. As only one side mirror is supported in order to prevent the distortions, no distortion of FP 7 appears very much and no degasification occurs even though a laser beam makes irregular reflection and strikes upon the U-shaped rings 27. Such a supporting system thus prevents contamination of the FP 7.