PURPOSE: To obtain the titled modulator capable of operating at a low voltage and in a broad-band by forming the optical modulator in a thin ferroelectric body through an epitaxial film composed of a MgAl2O4 film or a laminated film composed of said film and MgO film.

CONSTITUTION: The LiNbO3 film 4 is formed on a high resistance Si substrate through the epitaxial film of MgAl2O4 2 and and that of MgO 3. Ti diffusion optical waveguides 5 and 6, and a directional coupler 7 using said waveguides are formed in the film 4. A planar type electrodes 8 facing with a minute gap are formed on the optical waveguides of the directional coupler 7 through a buffer film 9 composed of SiO2. Thus, the film thickness of the film 4 is selected so as to be ≤3 times the gap of between the electrodes. A dielectric constant of the film 3 very small in comparison with the dielectric constant of the ferroelectric body. When the film thickness of the dielectric body is ≤3 times the gap of the electrodes, the value of a refractive index for modulated wave may be approached to the value of a refractive index for optical wave, compared with the dielectric body being only used to the substrate, and the modulation frequency band width may be magnified.