When forming electrodes for an optical waveguide element, a metal film is formed on a surface of a substrate, and openings of predetermined shapes are formed in the metal film. Then proton exchange is carried out on the surface of the substrate with the metal film used as a mask, and optical channel waveguides are thus formed. At least a part of edge portions of the metal film defining the openings is left on the substrate and the metal film is plated with plating metal. The metal film plated with the plating metal is processed into electrodes of predetermined shapes for applying an electric voltage to the optical channel waveguides.