An exhaust gas treatment apparatus comprising: a mercury oxidation apparatus that generates post-mercury oxidation exhaust gas from combustion apparatus exhaust gas generated by a combustion apparatus; a wet desulfurization apparatus that generates post-desulfurization exhaust gas and slurry from the post-mercury oxidation exhaust gas; a separation apparatus that generates gypsum and desulfurization drainage from the slurry; a first desulfurization drainage supply apparatus that supplies the desulfurization drainage to the combustion apparatus; a second desulfurization drainage supply apparatus that supplies the desulfurization drainage to the wet desulfurization apparatus; a halogen concentration sensor that measures a halogen concentration, which is the concentration of halogens contained in filtrate generated by the slurry being filtered; and a control apparatus that controls the second desulfurization drainage supply apparatus so that a supply amount, which is the amount of the desulfurization drainage supplied to the wet desulfurization apparatus, changes based wherein the combustion apparatus generates the combustion apparatus exhaust gas by combusting fuel together with the desulfurization drainage, wherein the post-mercury oxidation exhaust gas contains mercury oxidized by halogens contained in the combustion apparatus exhaust gas, and is desulfurized by the wet desulfurization apparatus using an absorbing liquid with which the desulfurization drainage has been mixed, and wherein the control apparatus further controls the first desulfurization drainage supply apparatus so that a supply amount, which is the amount of the desulfurization drainage supplied to the combustion apparatus, changes based on the halogen concentration. a mercury oxidation apparatus that generates post-mercury oxidation exhaust gas from combustion apparatus exhaust gas generated by a combustion apparatus; a wet desulfurization apparatus that generates post-desulfurization exhaust gas and slurry from the post-mercury oxidation exhaust gas; a separation apparatus that generates gypsum and desulfurization drainage from the slurry; a first desulfurization drainage supply apparatus that supplies the desulfurization drainage to the combustion apparatus; a second desulfurization drainage supply apparatus that supplies the desulfurization drainage to the wet desulfurization apparatus; a halogen concentration sensor that measures a halogen concentration, which is the concentration of halogens contained in filtrate generated by the slurry being filtered; and a control apparatus that controls the second desulfurization drainage supply apparatus so that a supply amount, which is the amount of the desulfurization drainage supplied to the wet desulfurization apparatus, changes based wherein the combustion apparatus generates the combustion apparatus exhaust gas by combusting fuel together with the desulfurization drainage, wherein the post-mercury oxidation exhaust gas contains mercury oxidized by halogens contained in the combustion apparatus exhaust gas, and is desulfurized by the wet desulfurization apparatus using an absorbing liquid with which the desulfurization drainage has been mixed, and wherein the control apparatus further controls the first desulfurization drainage supply apparatus so that a supply amount, which is the amount of the desulfurization drainage supplied to the combustion apparatus, changes based on the halogen concentration.