To improve corrosion-resistance of a furnace inner surface component against fused slag in a fusion furnace for heating waste combustion ash content to generate fused slag.

The fireproofing material constituting the inner surface of the fusion furnace is mainly composed of alumina. As the corrosion resistance reinforcing material for the fireproofing material, the surface of the fireproofing material is coated with zirconia contained material taking the raw material of the fireproofing material (mainly composed of alumina) as base material and containing 20 wt.% zirconia as base material. According to the invention, the corrosion resistance in the fused slag environment is more improved to lengthen the life of the fusion furnace as compared with only the conventional fireproofing material mainly composed of the alumina material.