A discharge gas treatment apparatus and method which lower treatment cost and which attain efficient removal of mercury contained in a discharge gas are provided. The discharge gas treatment apparatus, includes a cooling apparatus for controlling the temperature of the discharge gas to a predetermined temperature in accordance with the hydrogen chloride concentration of the discharge gas; an NO.sub.x removal catalyst unit for reducing nitrogen oxide contained in the discharge gas and for causing reaction between mercury and hydrogen chloride contained in the discharge gas, the discharge gas having been controlled to the predetermined temperature and having been introduced to the apparatus, with ammonia being added thereto; and a wet-format desulfurization apparatus, disposed on the downstream side with respect to the NO.sub.x removal catalyst unit, for removing sulfur oxide and mercury chloride contained in the discharge gas through dissolving sulfur oxide and mercury chloride in a liquid absorbent.