Wavelength tunable light source of external resonator type

An improvement is made to a wavelength tunable light source of an external resonator type for selecting a wavelength of light from a semiconductor laser (1) using a diffraction grating (3). The device has a prism (6,7,8) provided between the semiconductor laser (1) and the diffraction grating (3) for enlarging the beam shape of the light from the semiconductor laser (1) in the groove arrangement direction of the diffraction grating (3), emitting the light to the diffraction grating, and reflecting diffracted light from the diffraction grating to provide output light.