PURPOSE: To enable to oscillate with the finite spectrum width having different center wavelengths by a method wherein the beam of light emitted from a semiconductor laser is irradiated at the reflecting surface on the focus surface of a dispersing and spectral optical system, and the beam of light is fed back to the semiconductor laser again.

CONSTITUTION: The light of the wavelength λ1, reflected at the end section on the side of long wavelength of the reflecting surface 22, having a microscopic width, placed on the reflecting focus surface 2 of the dispersing and spectral optical system, is turned to parallel beams of light again by the lens 4, and irradiated to a diffraction grating 5 again at the angle β2. At this time, if an incidence angle is set at (α), a grating constant at (d) and an order of diffraction at (m), the wavelength λ1 on the side of the long wavelength is worked out as shown by formula II. And by repeating the reflection, the center wavelength of (λ1+λ2/2) and wavewidth of (λ1-λ2) are obtained for the oscillating light of the semiconductor laser.