Abstract: Synthetic macro-molecule materials are continuously supplied into the cylinder of an extruder so that said synthetic macro-molecule materials are internally heated by the shearing as well as kneading action of the screw to which they are subjected and externally heated by the heat transmitted through the wall of the cylinder of said extruder from an outside heat source so that said synthetic macro-molecule materials are continuously thermally decomposed within said cylinder, thereby vapourized thermally decomposed products are fed to the condensing means through one or more discharge orifices provided in said cylinder to be recovered there as thermally decomposed products having low molecular weights.