To explore the possibility that an electromagnetic induction heating coil is wound solenoidally around the trunk of a vertical decomposition tank and the energy of the inner wall of the decomposition tank to be red-heated by the heat of a reaction thereof with a magnetic flux is transferred directly to an object to be treated and utilized.

As a means for making best use of the IH reaction heat of the inner wall of the decomposition tank, waste plastics to be treated are pressed to the inner wall surface by sweptback blades, which are of an agitator incorporated in the decomposition tank and are retreated spirally along the inner wall, and waste plastics to be treated are dropped from an upper feed port on the surfaces of the sweptback blades and dried. The waste plastics to be treated, which are dropped on the bottom of the decomposition tank, are scooped up by the sweptback blades and brought into contact with the whole inner wall surface and reacted again by the action of the sweptback blades.