To permit recycling of a waste plastic which contains halogenated additives and has difficulty of material recycling due to mingling of impurities and different plastics, as a modifier for asphalt and adhesives, a fuel for heating, an additive for cutting oils and so on, without generation of halogenated dioxins, and thereby utilize the waste plastic.

The method consists of a chemical thermal decomposition process 5 and a distillation separation process 6. The process 5 consists of adding a fat and oil to a resin type waste containing a halogenated additive and decomposing the portion of synthetic resins of the waste thermally by a solvating thermal decomposition reaction under ordinary pressure to make the portion middle molecular and liquid so as to separate the insolubles, such as metal and inorganic matter, and thereby permit recycle of the halogen in the halogenated additive as a hydrogen halide without generation of dioxins. The process 6 consists of distilling the liquid portion from the process 5 under reduced pressure to separate into reusable light and heavy oil portions.