A mercury reduction system according to the present embodiment is a mercury reduction system that reduces NOx and Hg in flue gas discharged from a boiler, and includes a chemical supplying unit that mixes an NH4Cl solution, an NH3 solution, and an HCl solution in liquid states, and supplies a mixed solution into a flue provided downstream of the boiler, a reduction denitration apparatus that includes a denitration catalyst reducing NOx in the flue gas with NH3 and oxidizing Hg in the presence of HCl, and a wet desulfurization apparatus that reduces Hg oxidized in the reduction denitration apparatus with limestone-gypsum slurry.