Waste synthetic high polymer, especially those containing polyvinyl chloride and the like chlorine-containing synthetic resins are heated at 200.degree.-500.degree.C to thereby form a hot molten bath of fusible materials contained in the waste resin mixture and the chlorine-containing resins are effectively decomposed in the molten bath kept at the temperature of 200.degree.-500.degree.C with the generation of hydrogen chloride gas which can be recovered as hydrochloric acid.