A manufacturing method for an optical waveguide device. The manufacturing method includes the steps of forming an optical waveguide in a substrate having an electro-optic effect, forming an SiO.sub.2 film on the substrate, forming Si films on the SiO.sub.2 film, the lower surface of the substrate, and at least a part of the side surface of the substrate to thereby make a conduction between the Si film formed on the SiO.sub.2 film and the Si film formed on the lower surface of the substrate. The manufacturing method further includes the steps of applying a photoresist to the Si film formed on the SiO.sub.2 film, patterning the photoresist so that a portion of the photoresist corresponding to the optical waveguide is left, forming a groove on the substrate along the optical waveguide by reactive ion etching, and removing the photoresist and the Si films.