Abstract: In an external-cavity tunable wavelength light source using a semiconductor laser (LD), an oscillation wavelength is tuned by simultaneously changing a current to be injected to the phase adjustment area of the LD and the rotation angle of a diffraction grating while holding the same external cavity longitudinal mode. A light beam emitted from the AR-coated end face of the LD having the phase adjustment area is collimated by a lens and incident on the diffraction grating. Only a light component having a specific wavelength selected upon spectral dispersion is reflected by the diffraction grating and returns to the LD. With this operation, an external cavity (effective cavity length K) is formed between the LD and the diffraction grating, thereby performing laser oscillation at a wavelength determined by an external cavity longitudinal mode and the cavity loss of the diffraction grating. The effective cavity length K can be changed in accordance with the current to be injected to the phase adjustment area.