To eliminate the necessity of nitrogen gas.

An ash melting furnace 4 is connected to the downstream side of a rotary stoker incineration furnace 1 through an ash carrier conveyor 5. An economizer 9 is connected to a waste heat boiler 8 located above the secondary combustion chamber 1b of the rotary stoker incineration furnace 1, and a stack 16 is connected to the downstream side of the economizer through a flue gas line 10 having a gas temperature reducing tower 11, a bag filter 12, an induced draft fan 13 a flue gas reheater 14, and a catalytic reactor 15. A flue gas branch line 39 branched from the flue gas line 10 on the upstream side of the stack 16 is connected to an injection nozzle 37 installed in the ash carrier conveyor 5. When hot burned ash 3 taken out of the rotary stoker incineration furnace 1 is carried by the ash carrier conveyor 5, burned flue gas 7 with a low oxygen concentration led from the flue gas line 10 through the flue gas branch line 39 is blown into the conveyor to bring the atmosphere inside the ash carrier conveyor 5 to a low oxygen concentration to prevent the unburned combustibles in the burned ash 3 from being ignited.