A dry sorting machine having a separating function for removing solids of more than a predetermined size from ash and a drying function for drying the ash, and a magnetic sorting machine for removing metals from the dried ash are arranged in a pretreatment route for the ash to be supplied to the furnace. Further, there is provided a drying air pipe for introducing the high-temperature air heat-recovered from exhaust gases discharged from the ash melting furnace into the dry sorting machine. Thus, by the dry sorting machine having the separating and drying functions, simplification of the ash pretreatment process and reduction of the installation space can be achieved.