360.0 g of methyl ethyl ketone was added to a 2 liter three-necked flask provided with a stirrer, a thermometer, a reflux condenser, and a nitrogen gas introducing tube, followed by heating up to 75° C. Thereafter, while maintaining the temperature inside the flask at 75° C., a mixed solution of 180.0 g of phenoxyethyl acrylate, 162.0 g of methyl methacrylate, 18.0 g of acrylic acid, 72 g of methyl ethyl ketone, and 1.44 g of “V-601” (manufactured by Wako Pure Chemical Industries, Ltd.) was added dropwise thereto at a constant speed such that the dropwise addition was completed in two hours. After the completion of the dropwise addition, a solution containing 0.72 g of “V-601” and 36.0 g of methyl ethyl ketone was added thereto followed by stirring at 75° C. for 2 hours. Furthermore, a solution containing 0.72 g of “V-601” and 36.0 g of isopropanol was added thereto followed by stirring at 75° C. for 2 hours, and heated to 85° C. followed by stirring for another 2 hours, thereby obtaining a resin solution of a phenoxyethyl acrylate/methyl methacrylate/acrylic acid (=50/45/5 [mass ratio]) copolymer.
