PURPOSE: To improve modulating efficiency by forming an optical directional coupler of two layers of semiconductor layers sandwiching an insulator film.

CONSTITUTION: An InxGa1-xAsyP1-y optical waveguide layer 7 is grown to 1μm thickness, an SiO2 film 8 to 5,000 and an InxGa1-xAsyP1-x layer 9 to 1.5μm thickness, respectively, on an InP substrate 6 and a ridge part 10 is formed on the surface by etching. The propagating light is coupled via the film 8 to the layer 7 beneath said film when light is made incident on the ridge part 10 of the layer 9 on the surface having such structure. The ridge part 10 is formed in order to provide an effective refractive index distribution in a horizontal direction and to confine the propagaging light therein. Ohmic electrodes 11, 12 are then formed on the surface of the part 10 and the substrate 6 side and a bias is applied between the electrodes, by which the propagation constant of each optical waveguide is changed and the intensity of the exit light from each optical waveguide is controlled.