An exhaust gas treating apparatus removes nitrogen oxides and mercury in exhaust gas from a boiler using an ammonia denitration catalyst, including: an ammonium chloride powder feed unit for feeding ammonium chloride, in powder form, into a vicinity of an entrance of an economizer provided to a combustion gas flue of the boiler and/or an economizer bypass unit, the fed ammonium chloride in powder form being sublimed by a combustion gas, to thereby feed hydrogen chloride and ammonia into the flue; and an ammonium chloride liquid feed unit for feeding ammonium chloride, in liquid form, into a vicinity of the entrance of the economizer and/or the economizer bypass unit, the ammonium chloride liquid feed unit also being capable of feeding ammonium chloride, vaporization of the fed ammonium chloride in liquid form by the combustion gas also allowing hydrogen chloride and ammonia to be fed into the flue.