A mercury reduction system includes an NH4Cl supplying unit that sprays an NH4Cl solution into a flue of the boiler, a reduction denitration apparatus that includes a denitration catalyst for reducing NOx in the flue gas with NH3 and oxidizing Hg in the presence of HCl, and a wet desulfurization apparatus for reducing Hg oxidized in the reduction denitration apparatus with limestone-gypsum slurry. The NH4Cl supplying unit includes an NH4Cl solution feed pipe that supplies the NH4Cl solution into the flue in a liquid state, a blow pipe that is inserted into the flue so as to surround the NH4Cl solution feed pipe and through which air is injected into the flue, and a two-fluid nozzle that is fitted to an end of the NH4Cl solution feed pipe and through which the NH4Cl solution is injected. Through the two-fluid nozzle, the NH4Cl solution is injected and sprayed in fine liquid droplets. NH3 is supplied by an ammonia supplying unit and HCl is supplied by a hydrogen chloride supplying unit wherein both units are located between the NH4Cl agent supplying unit and the reduction denitration apparatus.