An exhaust gas processing system having excellent durability and good desulfurization and denitration efficiency is provided to efficiently recover carbon dioxide with high purity and reduced processing costs. The exhaust gas processing system has: a desulfurization unit removing sulfur oxides from the exhaust gas by the limestone-gypsum method; a denitration unit arranged downstream of the desulfurization unit to remove nitrogen oxides from the exhaust gas; a carbon dioxide recovery arranged downstream of the denitration unit to recover carbon dioxide from the exhaust gas; and an oxygen supply unit supplying to the desulfurization unit with a faction of the recovered gas from the carbon dioxide recovery unit as oxygen source. An analyzer is used to monitor the purity and recovery ratio of the carbon dioxide recovered, and the supplied ratio of recovered gas is adjusted, based on the monitored purity and recovery ratio.