PURPOSE: To facilitate variation of the spectral wavelength as well as improve its stability by a constitution wherein transparent elastic resin is interposed between a diffraction grating and a solid-state medium, and the angle of the diffraction grating can be varied.

CONSTITUTION: Transparent elastic resin 5 is interposed between a diffraction grating 6 and a solid-state medium 4, and the angle of the diffraction grating 6 can be varied by a micrometer head 7. The incident light 8 from an optical fiber 1 transmits through the solid-state medium 4, is converted into parallel rays by a concave reflecting mirror 3, passed through the solid-state medium 4 and the transparent elastic resin 5 and made incident on the diffraction grating 6. The diffracted light 9 is conversely passed through the transparent elastic resin 5 and the solid-state medium 4 and converged on an opticl fiber 2 by the concave reflecting mirror 3. Changing the angle of the diffraction grating 6 by turning the micromether head 7 permits the wavelength of the diffracted light 9 converged on the optical fiber 2 to be varied. Thereby, the spectral wavelength can be readily varied gas well as is stable because the optical system is integrated at all times.