PURPOSE: To enable the modulation of the intensity or phase of guided light to a phase at a high speed with the nonlinear optical device to be used for electronic equipment, optical communication fields, etc.

CONSTITUTION: A clad layer 4 of AlXGa1-XAs layers (X=0.51) is formed on a GaAs substrate 2. An optical waveguide layer 6 having the refractive index larger than the refractive index of the clad layer 4 is formed on the clad layer 4. The optical waveguide layer 6 is formed of the multiquantum well structure laminated with quantum wells of GaAs layers and barrier layers of the AlXGa1-XAs layers (X=0.51) in 20 periods. A clad layer 8 of the AlXGa1-XAs layers (X=0.51) having the refractive index smaller than the refractive index of the optical waveguide layer 6 is formed on the optical waveguide layer 6. The respective layers are so constituted that the line width A thereof is shorter than the wavelength of the guided light and is about 10 to 200nm line width.