A mercury content reduction system (10A; 10B; 10C) for reducing the content of nitrogen oxide and mercury in flue gas discharged from a boiler, the mercury content reduction system (10A; 10B; 10C) comprising: an oxidation-reduction agent supplying unit that comprises a spraying unit that sprays ammonium halide as an oxidation-reduction agent for producing hydrogen halide and ammonia when evaporated into a flue of the boiler in a liquid state; a reduction denitration apparatus that includes a denitration catalyst for reducing nitrogen oxide in the flue gas with ammonia and for oxidizing mercury in a presence of hydrogen halide; a wet desulfurization apparatus that reduces a concentration of the mercury oxidized in the reduction denitration apparatus with an alkali absorbent, characterised by also comprising an ammonia supplying unit that supplies ammonia into the flue and a hydrogen halide supplying unit that supplies hydrogen halide into the flue, both of which are provided between the spraying unit of the oxidation-reduction agent supplying unit and the reduction denitration apparatus; nitrogen oxide concentration meters (62-1, 62-2) that are provided upstream and downstream of the reduction denitration apparatus, each of which measures a concentration of nitrogen oxide in the flue gas, a mercury concentration meter that is provided downstream of the reduction denitration apparatus and measures a concentration of Hg contained in the flue gas; and a hydrogen halide concentration meter that is provided downstream of the reduction denitration apparatus and measures a concentration of hydrogen halide contained in the flue gas, wherein at least one of the required hydrogen halide and NH 3 is supplied by the hydrogen halide supplying unit and the ammonia supplying unit if a hydrogen halide concentration required for oxidizing Hg is different from a NH 3 concentration required for reducing NO x, and wherein an oxidation ratio of Hg in the reduction denitration apparatus is identified by using each of measured concentration values of the mercury concentration meter and the hydrogen halide concentration meter, and a supply amount of hydrogen halide supplied from hydrogen halide supplying unit is controlled with measured concentration values. an oxidation-reduction agent supplying unit that comprises a spraying unit that sprays ammonium halide as an oxidation-reduction agent for producing hydrogen halide and ammonia when evaporated into a flue of the boiler in a liquid state; a reduction denitration apparatus that includes a denitration catalyst for reducing nitrogen oxide in the flue gas with ammonia and for oxidizing mercury in a presence of hydrogen halide; a wet desulfurization apparatus that reduces a concentration of the mercury oxidized in the reduction denitration apparatus with an alkali absorbent, characterised by also comprising an ammonia supplying unit that supplies ammonia into the flue and a hydrogen halide supplying unit that supplies hydrogen halide into the flue, both of which are provided between the spraying unit of the oxidation-reduction agent supplying unit and the reduction denitration apparatus; nitrogen oxide concentration meters (62-1, 62-2) that are provided upstream and downstream of the reduction denitration apparatus, each of which measures a concentration of nitrogen oxide in the flue gas, a mercury concentration meter that is provided downstream of the reduction denitration apparatus and measures a concentration of Hg contained in the flue gas; and a hydrogen halide concentration meter that is provided downstream of the reduction denitration apparatus and measures a concentration of hydrogen halide contained in the flue gas, wherein at least one of the required hydrogen halide and NH 3 is supplied by the hydrogen halide supplying unit and the ammonia supplying unit if a hydrogen halide concentration required for oxidizing Hg is different from a NH 3 concentration required for reducing NO x, and wherein an oxidation ratio of Hg in the reduction denitration apparatus is identified by using each of measured concentration values of the mercury concentration meter and the hydrogen halide concentration meter, and a supply amount of hydrogen halide supplied from hydrogen halide supplying unit is controlled with measured concentration values.