A distributed feedback laser diode comprises a phase-shifting portion in diffraction gratings. The magnitude of a phase shift in the phase-shifting portion is 8.LAMBDA./40 to 9.LAMBDA./40, .LAMBDA. representing twice the distance between the diffraction gratings. A main mode stands on a lower wavelength side than the center of a stop band when an injected current is of a level of a threshold current, whereas the main mode is shifted to the center of the stop band and a sub mode is suppressed from growing when the injected current is of a level of an operating current.