In a semiconductor optical modulator of this invention, each quantum-well layer and each barrier layer of a quantum-well structure serving as a light absorption layer are respectively made of In_1-X-YGa_XAl_YN (0X, Y1, 0X+Y1) and In_1-X-YGa_XAl_YN (0X, Y1, 0X+Y1). An electric field is being generated in the light absorption layer by spontaneous polarization.