A polymeric optical waveguide substrate includes a first region provided thereon with a polymer optical waveguide and an optical waveguide-free second region, the first and second regions being on the same substrate, wherein an adhesive layer is between the lowermost polymer layer constituting the optical waveguide in the first region and the substrate, and wherein a groove intrudes into the substrate at the boundary between the first and second regions. A method for preparing the waveguide substrate includes the steps of applying an adhesive layer onto the first region while the second region is free of any adhesive layer; applying a polymer onto the whole surface of the substrate to give a polymer layer for forming an optical waveguide following the formation of the adhesive layer; cutting the polymer layer at the boundary between the first and second regions; and peeling off or removing the polymer on the second region.