A sodium-based dechlorinating agent is added to a flue gas; hydrogen chloride contained in this flue gas is removed as residue of dechlorination; the thus removed residue of dechlorination is dissolved by adding water; water-insoluble constituents are separated from the resulting aqueous solution; and after adjusting pH of the aqueous solution remaining after separation of the water-insoluble constituents, mercury, dioxin, and the like are removed and discharged. The sodium-based dechlorinating agent is mixed with a hydrophilic anti-caking agent, with an angle of repose of 40° or more, a dispersibility of less than 50, and a floodability index value of less than 90. This permits inhibition of occurrence of a pressure drop and leakage in the filter cloth of the dust collector.