To prevent reduction of power of output light through spatial hole burning (carrier density decrease) caused in a /4 phase shift DFB laser.

The /4 phase shift DFB laser includes: an active layer 40 for generating stimulated emission light; a diffraction grating layer 30 formed on or under the active layer 40 for attaining a single longitudinal mode; and upper clad layers 41 and 41a and a lower clad layer 21 laminated with the active layer 40 and the diffraction grating layer 30 in between from the upper and lower directions. In the diffraction grating layer 30, a diffraction grating formation region 31, in which an uneven diffraction grating 33 is formed, and a diffraction grating non-formation region 32, in which the diffraction grating 33 is not formed, are formed by turns periodically in a range affected by an evanescent field of the stimulated emission light, which is subjected to wave guide in the active layer 40.