Abstract: A technique of preparing oil from waste plastics, especially the one to which a mechanism of an extrusion molding machine is applied, is provided wherein an effective control of a decomposition process is conducted in order to effectively prevent the generation of carbon and to efficiently obtain recovered products having a desired composition. For this purpose, reactors 1a, 1b, and 1c having built in carrying means 3a, 3b, and 3c, respectively are used as connected in stages, in which reactors waste plastics are heated and decomposed into oil as carried. One or a series of reactors are used as a unit to form a melting zone and a decomposing zone in the carrying direction.