To provide a polarization modulation semiconductor laser having such structure as canceling the spatial fluctuation of refractive index in a resonator due to hole burning effect and an active layer of different combination of well layers.

A distributed feedback semiconductor laser having a diffraction grating 102 is provided with two or more electrodes 116, 111 in the direction of resonator wherein first and second optical waveguide regions 11, 12 corresponding to the electrodes and constituting the resonator can be injected with a current independently. Active layers 130, 131 in two optical waveguide regions comprising a plurality of quantum wells have one or more strain free or compression strained quantum well and a tension strained quantum well, respectively, and the combination of the number of wells is different between the optical waveguide regions 11, 12. Optical guide layers 120, 121, 122, 123 formed between the active layers 130, 131 and the diffraction grating 102 on a semiconductor substrate are thin at least in the central part and thick at least on one end part of resonator.