For example, the masterbatch and the oxidizable polyamide (A) are fed to at least one melt-kneader of a molding machine having one or more melt-kneaders in the form of dry blend or respectively from matering feeders, melt-kneaded therein, and then formed into molded articles of various shapes, for example, preforms such as film, sheet, tube and parison for producing packaging containers, and packa