To carry out melting treatment of flying ash without taking a countermeasure to separate flying ash and desalinized flying ash by a two stage bag filter.

Waste material 1 is burnt in an incinerator 2 and incinerator bottom ash 1a is directly casted in a melting furnace 3. Flying ash 1d in combustion exhaust gas generated in the incinerator 2 is collected by the bag filter 6. At this time, sodium system antichlor 9 is blown in an inlet of smoke passage of the bag filter 6 and hydrochlrolide in the combustion exhaust gas is neutralized and removed on a filter cloth surface of the bag filter 6. Consequently, flying ash 1d is collected as desalinized flying ash which contains sodium chloride as a principal component. The desalinized flying ash 1f is subjected to melting treatment with the incinerator bottom ash 1a in the furnace3 to volatilize sodium chloride.