The air pollution control device includes: NH4Cl solution supply means 16 for spraying an NH4Cl solution 14 by a plurality of spray nozzles 15 into a flue gas duct 13 at the downstream of the boiler 11; a reduction-denitration device 18 including a denitration catalyst for reducing NOx in the flue gas 12 with NH3 and for oxidizing Hg under the coexistence with HCl; and a wet desulfurization device 22 for reducing the amount of Hg oxidized in the reduction-denitration device 18 by using a limestone-gypsum slurry 21. The NH4Cl solution supply means 16 supplies the NH4Cl solution 14 from the spray nozzles 15 so as to prevent the NH4Cl solution 14 from being adhered to an inner wall of the flue gas duct 13 through which the flue gas 12 is flowing.