PURPOSE: To provide a combustion discharged gas toxic substance thermal decomposition furnace in which the toxic gas discharged from an existing furnace is thermally decomposed to enable itself to be non-harmful state and its energy efficiency is superior.

CONSTITUTION: Longitudinal U-shaped heaters 6 are arranged along a side wall 1, within a closed cylindrical space comprised of the side wall 1, a lower wall 2, an upper edge member 3 and an upper lid. The heater 6 is of an electrical resisting heater having as its major substance molybdeum desilicate. The heaters 6 are connected in parallel from each other and they can be separately and optionally fixed or removed. The discharged gas fed from the waste material combustion furnace through one discharged gas pipe is distributed into a plurality of heat-resistant pipes 7. The heaters 6 generate heat at a high temperature of about 1900°C and the discharged gas within the heat resistant pipes 7 is also heated up to a high temperature more than 1400°C. Toxic substances such as dioxine in the discharged gas are thermally decomposed due to their high temperature and changed into non-toxic substances.