An object of the present invention is to provide a treatment method and a plant of an exhaust gas in which corrosion resistance of the exhaust gas treatment plant to a sulfuric acid mist after wet type desulfurization is increased. The treatment method of an exhaust gas in the present invention, wherein sulfur oxide is removed by wet type desulfurization of the exhaust gas and a sulfuric acid mist is removed by feeding ammonia into the exhaust gas, is characterized in that an ammonia gas is mixed into the exhaust gas by feeding inorganic ammonium salt to an alkali desulfurizing agent when the desulfurizing agent is sprayed into the exhaust gas to absorb and remove the sulfur oxide in the exhaust gas.