PURPOSE: To prevent denatured layer formation in a part where an optical waveguide is to be formed by making the thickness of a film formed in a first process by thicker than the thickness necessary to form a waveguide in a second process wherein the film is so formed in the first process as to retain a part of the film which remains due to insufficient diffusion on a substrate.

CONSTITUTION: A Ti film 2 is formed on a LiNbO3 substrate 1 and the substrate 3 and the Ti film 2 are heated to diffuse Ti of the Ti film 2 in the substrate 1 and form a channel-type optical waveguide 3. A part of the Ti film 2 which is not diffused sufficiently remains on the substrate 1 and Ti film 2 is converged into TiOx film 102 due to the oxygen atmosphere at the time of diffusion and the Ti film 2 remaining on the substrate 1 is removed finally by etching by a HF solution and thus a channel-type single mode optical waveguide 3 is completed. In this case, the surface area of a part where the optical waveguide 3 to be formed is covered with the TiOx film 102 till the diffusion is completed. At the time of forming the Ti film 2, the thickness of the film is made to be thicker than the found out minimum thickness.