A collimator lens converts light beams emitted from a low reflectance facet of a semiconductor laser into parallel light beams. A diffraction grating receives and diffracts the light beams at a predetermined incident position and a predetermined angle of a diffraction face. A turnable mirror has a mirror surface which is positioned opposite the diffractance facet of the diffraction grating, makes the diffracted light beams incident in a reverse optical path to the diffraction face, and returns the incident light beams to the laser. A stationary mirror makes the light beams from the collimator lens incident from a predetermined direction to a predetermined position of the diffraction grating at the side of the mirror surface of a virtual plane extending from the mirror surface of the mirror and at the side of the diffraction face of a virtual plane extending from the diffraction face of the diffraction grating.