An exhaust gas treatment system (10A; 10B) for exhaust gas from a reduction device, comprising: a reduction device configured to perform a reduction process to turn iron oxide to reduced iron; a CO 2 recovery device (21;1000) that includes a CO 2 absorbing tower configured to recover CO 2 with CO 2 absorbing liquid from exhaust gas containing at least iron-based solid substances and CO 2, which are discharged from the reduction device, and an absorbing liquid regenerating tower configured to regenerate lean solution having low CO 2 concentration by diffusing CO 2 from rich solution having absorbed CO 2, through heat exchange with steam from a reboiler, the rich solution being extracted from the CO 2 absorbing tower and introduced to the absorbing liquid regenerating tower side through a rich solution supply line (L 2; L 11); a lean solution supply line (L 12) through which the lean solution regenerated in the absorbing liquid regenerating tower is extracted from the absorbing liquid regenerating tower and is introduced to the CO 2 absorbing tower to be reused in the CO 2 absorbing tower; a branch liquid line (L 21) branched from the lean solution supply line (L 12); a recovery device collector that is provided to the branch liquid line (L 21) and is configured to collect the iron-based solid substances contained in the lean solution (1005B-1) with a filter (51; 55); a liquid filtrate supply line (L 3; L 22) for returning liquid filtrate (1005B-2) passing through the recovery device collector to the lean solution supply line (L 12); and a removed substance returning line (L 4) for returning recovery device removed substances, which contain the iron-based solid substances that are collected by the recovery device collector and removed by a desorption device, to the reduction device side. a reduction device configured to perform a reduction process to turn iron oxide to reduced iron; a CO 2 recovery device (21;1000) that includes a CO 2 absorbing tower configured to recover CO 2 with CO 2 absorbing liquid from exhaust gas containing at least iron-based solid substances and CO 2, which are discharged from the reduction device, and an absorbing liquid regenerating tower configured to regenerate lean solution having low CO 2 concentration by diffusing CO 2 from rich solution having absorbed CO 2, through heat exchange with steam from a reboiler, the rich solution being extracted from the CO 2 absorbing tower and introduced to the absorbing liquid regenerating tower side through a rich solution supply line (L 2; L 11); a CO 2 absorbing tower configured to recover CO 2 with CO 2 absorbing liquid from exhaust gas containing at least iron-based solid substances and CO 2, which are discharged from the reduction device, and an absorbing liquid regenerating tower configured to regenerate lean solution having low CO 2 concentration by diffusing CO 2 from rich solution having absorbed CO 2, through heat exchange with steam from a reboiler, the rich solution being extracted from the CO 2 absorbing tower and introduced to the absorbing liquid regenerating tower side through a rich solution supply line (L 2; L 11); a lean solution supply line (L 12) through which the lean solution regenerated in the absorbing liquid regenerating tower is extracted from the absorbing liquid regenerating tower and is introduced to the CO 2 absorbing tower to be reused in the CO 2 absorbing tower; a branch liquid line (L 21) branched from the lean solution supply line (L 12); a recovery device collector that is provided to the branch liquid line (L 21) and is configured to collect the iron-based solid substances contained in the lean solution (1005B-1) with a filter (51; 55); a liquid filtrate supply line (L 3; L 22) for returning liquid filtrate (1005B-2) passing through the recovery device collector to the lean solution supply line (L 12); and a removed substance returning line (L 4) for returning recovery device removed substances, which contain the iron-based solid substances that are collected by the recovery device collector and removed by a desorption device, to the reduction device side.