PURPOSE: To detoxify a combustible waste material of every kind without generating smoke and a malodor by igniting carbide remaining in a hermetically closed treatment tank in a process thermally decomposing a combustible waste material in the treatment tank with air introduced into the bottom part of the treatment tank.

CONSTITUTION: At a point of time when carbide residue is formed by the thermal decomposition of the waste material F in an inside container 1 being a treatment tank in a thermal decomposition treatment process, the heating by the heating burner 5 of the inside container 1 is stopped while a suction means 9 is operated and, at the same time, the valve 1c of an introducing pipe 1b is opened and the open air is introduced into the inside container 1 by the suction force of a suction means 9 to partially ignite carbide residue to burn a part thereof. By this constitution, carbide in the inside container 1 is reduced in vol. by partial combustion and, at the same time, the thermal decomposition of unburnt carbide is generated by combustion heat to form combustible gas which is, in turn, recovered by the suction force of the suction means.