An optical waveguide substrate includes: a substrate; a clad disposed on a plane of the substrate and made of a transparent material; and a plurality of cores that are surrounded by the clad, extend in parallel with the plane of the substrate and are made of a transparent material having a refractive index different from a refractive index of the clad, the cores including at least a pair of cores with diameters different from each other. The cores are provided at positions where centers of sections of the cores are all positioned in a straight line.