To provide a thermally decomposing treatment system capable of suppressing deposition and solidification of plastic in a thermally decomposing device without separately preparing a deposition preventing medium such as sand when subjecting car shredder dust, in particular, to thermally decomposing treatment.

By casting molten slag produced in a combustion melting furnace 3' into a rotary kiln 2 together with ASR, the molten slag functions as a scraper to scrape off plastic already deposited inside of the rotary kiln 2. Consequently, plastic can be prevented from being remarkably deposited and solidified inside of the rotary kiln 2 when subjecting ASR containing a large amount of plastic to thermally decomposing treatment.