Abstract: An improved system for treating waste material in a molten state wherein the waste material in the form of finely pulverized combustible waste material including incombustible material, e.g., pulverized coal, dried sludge derived from sewerage or the like is burnt in a primary combustion furnace and then burnt in a secondary combustion furnace, the incombustible material is molten to form a flow of molten slag and the resultant molten slag is taken to the outside of the system is disclosed. Floatable dust in the combustion waste gas is collected and agglomerated in the atmosphere of a swirling flow having a high temperature enough to keep the incombustible material in a molten state, whereby a flow of molten slag is produced and then it is cooled so as to allow it to be discharged to the outside of the system. The molten slag may flow back by its own gravity weight to the combustion furnace against a counterflow of the waste gas.