Conventional methods for treating waste and ash generated in incineration furnaces are to melt and solidify the ash at high temperature. Though various systems have been installed and operated, they have some critical problems with their high operation cost including fuel cost, technical difficulties in melting mixed materials contained in ash, and practical incapability in melting iron materials and clinkers. The system proposed here is designed to reduce fuel consumption by utilizung the combustion heat of unburned carbonaceous materials contained in discharged ash from a waste incineration furnace to assist melting the incineration ash. The furnace system has oxygen burners for burning carbon and for melting iron materials and clinkers. Test operation in a 12 t-ash/day test plant revealed that the proposed system is capable of melting the ash containing 20-30 wt% of iron and clinker rumps of the size of around phi 15 cm without auxiliary fuel supply other than unburned carbon residue in ash.