An optical waveguide is provided and includes: a core forming layer with a high refractive index; and a first clad layer with a low refractive index, bonded to a first main surface of the core forming layer. The core forming layer is provided in its plane direction with a core portion, lateral clad portions each having one side adjacent to a corresponding side of the core portion, and high refractive index portions each adjacent to the other side of a corresponding one of the lateral clad portions. The core portion is provided in its plane direction with a central region, and GI regions in each of which a refractive index continuously decreases from the central region toward an interface with the corresponding one of the lateral clad portions. The lateral clad portions each include a region having a constant refractive index.