PURPOSE: To provide a distribution reflective type semiconductor light source which oscillates in a single vertical mode and whose spectrum line width is narrow even when the wavelength is variable.

CONSTITUTION: An optical fiber 4 having specified length is connected to the light-emitting plain of a wavelength-variable distribution reflective type semiconductor laser 1a. A part of the light-emitted from the distribution reflective laser 1a and entering the optical fiber 4 becomes backward scattering light due to the structural unevenness that the optical fiber 4 itself possesses and automatically becomes a return light to the distribution reflective type semiconductor laser 1a, making the spectrum line width narrow.