Abstract: The flow of waste gas is directed downwardly by means of a refractory guide chute so as to follow the down flow of molten slag at the downstream end of a slag outlet. Thus the sensible heat of the high-temperature waste gas forcedly heats the molten slag and the downstream end portion of the slag outlet. Consequently, it becomes less possible for the slag outlet to be choked up. When there are signs that the slag outlet is being choked up, the plasma torch is slightly tilted to the upstream end of the slag outlet so as to stabilize the discharge of the molten slag. A housing or jacket is disposed circumferentially around an incineration residue feed pipe, inserted through the incineration residue inlet, and adapted to allow an annular gas curtain to issue therefrom. Fuel injection against the waste gas in the furnace body is particularly suitable for decreasing the amount of NOx.