An optical functional device comprises a dielectric substrate 5, a ferroelectric thin layer 10 provided on the dielectric substrate 5 and comprising a material having electro-optical effect and an electrode 3A, 3B provided on the ferroelectric thin layer 10. A part of the ferroelectric thin layer 10 functions as a core 9 of the optical wave guide and the dielectric substrate functions as a clad for the optical waveguide. The optical waveguide 9 constitutes a multi-mode waveguide in the direction "D" of depth of the ferroelectric thin layer.