An optical waveguide device is disclosed that includes a lower clad layer of a polymer resin material formed on a substrate, a core of a polymer resin material formed on the lower clad layer, and an upper clad layer of a polymer resin material covering the core. The core surrounded by the lower and upper clad layers forms an optical waveguide. The core has first and second opposing surfaces, the first surface facing the lower clad layer and the second surface facing away from the lower clad layer. The core is shaped so that the width of the first surface is greater than the width of the second surface in a cross section of the core.