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

A sorting device 7 takes metal residues recyclable outside the system out of thermally decomposed residues produced in a rotary kiln 2, and a sorting device 8 takes finely granular carbon residues highly usable as fuel out of the remaining thermally decomposed residues. The remaining thermally decomposed residues are charged into the rotary kiln 2 together with ASR to function as the deposition preventing medium for plastic in the rotary kiln, while at least part of the thermally decomposed residues is circulated to easily and inexpensively suppress the deposition and solidification of the plastic in the rotary kiln 2 without separately getting the deposition preventing medium from the outside of the thermal depositing treatment system 1.