A process for producing a polymer optical waveguide including the steps of: preparing a mold by applying a mold-forming resin layer onto a master template, peeling the layer from the master template to obtain a template, and cutting both ends of the template to expose a concave portion; bringing the mold into close contact with a film used for a cladding layer; introducing, by capillarity, a UV-curable resin or heat-curable resin by contacting the resin with one end of the mold; curing the introduced resin and removing the mold from the film; and forming a cladding layer on film on which the core has been formed, wherein a sectional area, a sectional shape, or both of a sectional area and a sectional shape of the core changes in a longitudinal direction of the core, and both end faces of the core have different areas.