A wavelength-tunable semiconductor laser having an active part and an external waveguide part is disclosed. The external waveguide part has a diffraction grating, such as a distributed Bragg reflector, and a quantum well structure optically coupled to the diffraction grating, together with electrodes for impressing an electric field to the quantum well structure. By applying the electric filed to the quantum well structure, the refractive index in the optically coupled diffraction grating/quantum well structure, and light oscillation wavelength in the external waveguide part, can be varied.