To minimize a total amount of an additive and a fusion material without decreasing a salt constituent evaporation rate by a method wherein a stoichiometric relation of an alkali metal-containing additive and a calcium- containing additive is so adjusted as to satisfy the specific formulas in cleaning flue gas.

An alkali metal-containing additive and a calcium-containing additive together with flue gas are blown into a reaction tower 4 connected to a waste gas side of a boiler 2. For example, lime in a Ca(OH)2 powder shape and NaHCO3 powder are used. At that time a stoichiometric relation of both is so adjusted as to satisfy formulas I and II. Acid gases HCl and SO2 in the flue gas react with lime and NaHCO3 with the additives to form CaSO3, CaCl, Na2SO4, and NaCl, and they together with flue gas dust are separated in a dust precipitator 5. Those together with the dust in flue gas are sent to a melting furnace 8. Thereby, an efficiency of a melting process can be improved.