An exhaust gas treatment device (10A; 10B; 10C; 10D), comprising: a nitrogen oxide absorbing unit configured to absorb and remove nitrogen oxides in exhaust gas with nitrogen oxide absorbing liquid by introducing the exhaust gas, which is discharged from a combustion device and contains at least nitrogen oxides and carbon dioxide; an exhaust gas discharge line (L 3) connected to the nitrogen oxide absorbing unit, for discharging treated exhaust gas from which nitrogen oxides in the exhaust gas are removed; a nitrogen oxide absorbing liquid circulation line for circulating the nitrogen oxide absorbing liquid, the nitrogen oxide absorbing liquid circulation line connecting a bottom part and a top part of the nitrogen oxide absorbing unit; a nitrogen oxide absorbing liquid extraction line (L 5) for extracting the nitrogen oxide absorbing liquid, which is circulating, the nitrogen oxide absorbing liquid extraction line (L 5) branched from the nitrogen oxide absorbing liquid circulation line (L 4); a nitrogen oxide absorbing liquid heating/regenerating unit configured to obtain released gas containing at least nitrogen monoxide and carbon dioxide and nitrogen oxide absorbing liquid regenerated liquid by subjecting the nitrogen oxide absorbing liquid, which is extracted through the nitrogen oxide absorbing liquid extraction line, to heating and regeneration treatment; a released gas line (L 6) connecting the nitrogen oxide absorbing liquid heating/regenerating unit and the exhaust gas discharge line (L 3), for introducing the released gas from the nitrogen oxide absorbing liquid heating/regenerating unit to the exhaust gas discharge line (L 3); and a regenerated liquid discharge line (L 7) connecting the nitrogen oxide absorbing liquid heating/regenerating unit and the nitrogen oxide absorbing liquid circulation line, for introducing the nitrogen oxide absorbing liquid regenerated liquid from the nitrogen oxide absorbing liquid heating/regenerating unit to the nitrogen oxide absorbing liquid circulation line (L 4), wherein the exhaust gas discharge line (L 3) is connected to a CO 2 absorbing column of a CO 2 recovery device, for feeding the treated exhaust gas together with the released gas to the CO 2 absorbing column. a nitrogen oxide absorbing unit configured to absorb and remove nitrogen oxides in exhaust gas with nitrogen oxide absorbing liquid by introducing the exhaust gas, which is discharged from a combustion device and contains at least nitrogen oxides and carbon dioxide; an exhaust gas discharge line (L 3) connected to the nitrogen oxide absorbing unit, for discharging treated exhaust gas from which nitrogen oxides in the exhaust gas are removed; a nitrogen oxide absorbing liquid circulation line for circulating the nitrogen oxide absorbing liquid, the nitrogen oxide absorbing liquid circulation line connecting a bottom part and a top part of the nitrogen oxide absorbing unit; a nitrogen oxide absorbing liquid extraction line (L 5) for extracting the nitrogen oxide absorbing liquid, which is circulating, the nitrogen oxide absorbing liquid extraction line (L 5) branched from the nitrogen oxide absorbing liquid circulation line (L 4); a nitrogen oxide absorbing liquid heating/regenerating unit configured to obtain released gas containing at least nitrogen monoxide and carbon dioxide and nitrogen oxide absorbing liquid regenerated liquid by subjecting the nitrogen oxide absorbing liquid, which is extracted through the nitrogen oxide absorbing liquid extraction line, to heating and regeneration treatment; a released gas line (L 6) connecting the nitrogen oxide absorbing liquid heating/regenerating unit and the exhaust gas discharge line (L 3), for introducing the released gas from the nitrogen oxide absorbing liquid heating/regenerating unit to the exhaust gas discharge line (L 3); and a regenerated liquid discharge line (L 7) connecting the nitrogen oxide absorbing liquid heating/regenerating unit and the nitrogen oxide absorbing liquid circulation line, for introducing the nitrogen oxide absorbing liquid regenerated liquid from the nitrogen oxide absorbing liquid heating/regenerating unit to the nitrogen oxide absorbing liquid circulation line (L 4), wherein the exhaust gas discharge line (L 3) is connected to a CO 2 absorbing column of a CO 2 recovery device, for feeding the treated exhaust gas together with the released gas to the CO 2 absorbing column.