It is provided a structure capable of reducing the loss of incident light propagating in a three-dimensional optical waveguide in a optical waveguide substrate having a ridge type optical waveguide. An optical waveguide substrate has a ferroelectric layer 3 made of a ferroelectric material, a ridge portion formed on a surface of the layer 3 and protrusions provided in both sides of the ridge portion. A three-dimensional optical waveguide is provided in the ridge portion. Alternatively, the substrate has a ridge portion 15 formed on a surface of the layer 3 and step portions provided in both sides of the ridge portion 15 and lower than the ridge portion 15. Grooves 16 are formed in the outsides of the step portions, respectively, and a three-dimensional optical waveguide is provided in the ridge portion 15.