To remove molten fly ash fixed in a duct, without stopping an operation by providing a heater for holding an exhaust gas at a specific temperature or above in an exhaust gas duct to a cooling tower for cooling exhaust gas generated from a melting furnace, and providing at least two systems of the ducts and the towers.

A melting furnace 1 for obtaining a slag A by melting and solidifying an incinerating residue generates an exhaust gas B containing a molten fly ash when operated, and introduces the gas B to a cooling tower 7 via an exhaust gas duct 2 for cooling the gas B. In this case, branching valves 6, 6' are provided to branch the duct 2 into two systems at a prescribed position of the duct 2. Furthermore, a heater 5 is provided to hold an exhaust gas temperature around the duct 2 from the furnace 1 to the valve 6 at 800°C or higher to prevent adherence of the ash to the duct 2 of this portion. If adherence of the ash to the tower 7 of the one system occurs, and to remove this, the tower 7 of the other system is set to an operating state, and the one system is repaired.