An optical waveguide includes a layer A and a plurality of cores enclosed in a cladding. During production of the optical waveguide, a layered film including alternate layers of a core layer and a cladding layer is cut so as to form a groove that penetrates through the layered film in a thickness direction and so as to form a plurality of core portions, and the layer A is provided so as to partially fill the groove depthwise and so as to maintain spacing between the plurality of core portions before the core portions is enclosed by the cladding.