The present invention provides a process for producing a polymer optical waveguide, comprising the steps of: preparing a mold comprising a concave portion; bringing a substrate into contact with the mold, filling the concave portion with a core-forming curable resin; curing the core-forming curable resin in the concave portion; removing the mold from the substrate; and forming a clad layer on the substrate; wherein the mold comprises a cured resin layer and a reinforcing member, the cured resin layer has the concave portion, the reinforcing member has an introduction portion which communicates with the concave portion and an injection inlet which communicates with the introduction portion, and a discharge portion which communicates with the concave portion is provided in the reinforcing member, the cured resin layer and/or a space between the cured resin layer and the reinforcing member, and a resin injecting device which is used in the process.