A kink-free semiconductor optical device stabilizing laser oscillation and producing a high optical performance. The semiconductor optical device includes a beam waveguide extending in a longitudinal direction between a pair of end surfaces. The beam waveguide includes an active layer having a quantum well structure with at least one well layer and two barrier layers, and a pair of cladding layers sandwiching the active layer. The active layer has first and second regions in the longitudinal direction, the photon density in the first region being larger than in the second region. The first region has a differential gain greater than the second region so that variation of refractive index across the beam waveguide is reduced.