<p>Disclosed is an optically functional device comprising a dielectric substrate (5), a ferroelectric thin layer (10) which is arranged on the dielectric substrate (5) and composed of a material having electro-optic effects, and electrodes (3A, 3B) arranged on the ferroelectric thin layer (10). By having a part of the ferroelectric thin layer (10) function as the core (9) of an optical waveguide and having the dielectric substrate (5) function as the cladding of the optical waveguide, the optical waveguide (9) is constituted as a multimode waveguide at least in the direction of depth (D) of the ferroelectric thin layer (10).</p>
