To provide a waste plastic decomposition apparatus that can inhibit the occurrence of carbon accompanied by decomposition reactions by enabling prompt and efficient separation of vaporized components formed in the liquid- phase polymer from the liquid phase polymer, and can efficiently recover the vaporized components with a desired molecular weight and can perform the decomposition of waste plastics in a valid cost.

The decomposition chamber for heating the polymer of liquid phase is made to be an upright cylinder and a rotor 10 that has a shape almost corresponding to the shape of the inner surface of the decomposition chamber is set inside the chamber. Further, spiral screws 12 are grooved on the outer face of the rotor 10 to form the reaction space 11 between the inner face of the decomposition chamber and the outer face of the rotor 10. As the liquid phase polymer flowing down is screwed up with the screw 12, the thermal decomposition is continued in the reaction space 11.