A combustion exhaust gas treatment system comprising: a combustion apparatus, generating an exhaust gas, an air heater for recovering heat contained in the exhaust gas from the combustion apparatus and for applying the heat recovered by the air heater into combustion air to be supplied to the combustion apparatus, a first heat exchanger for recovering heat contained in exhaust gas from the air heater and for applying the heat recovered by the first heat exchanger into a heat medium, an electrostatic precipitator for electrostatic precipitation treatment of exhaust gas from the first heat exchanger, a wet desulfurization device for wet desulfurization of exhaust gas from the electrostatic precipitator, a pre-scrubber for pre-scrubbing exhaust gas from the wet desulfurization device, an absorption column for receiving exhaust gas from the pre-scrubber for bringing the exhaust gas into contact with an absorbent to obtain a CO 2 absorbed absorbent and a CO 2 removed gas, a desorption column for releasing CO 2 from the CO 2 absorbed absorbent, to obtain a regenerated absorbent in a liquid state, a reboiler for vaporizing regenerated absorbent in a liquid state to generate regenerated absorbent in a gas state which supplies heat to the desorption column, a flash tank for flash vaporizing the regenerated absorbent recovered in a liquid state from the reboiler, a vapor recompression apparatus for compressing vapor withdrawn from an upper part of the flash tank to be fed to the desorption column, and a second heat exchanger for transferring heat to the CO 2 absorbed absorbent from the regenerated absorbent withdrawn in the liquid state from the flash tank; the CO 2 absorbed absorbent being able to be supplied from the absorption column to the desorption column via the second heat exchanger, and the regenerated absorbent being able to be supplied from the desorption column to the absorption column via the flash tank and the second heat exchanger in this order, wherein the system further comprises a third heat exchanger provided between the second heat exchanger and the desorption column to apply the heat recovered by the heat medium to the CO 2 absorbed absorbent and/or a fourth heat exchanger provided between the flash tank and the second heat exchanger to apply the heat recovered by the heat medium to the regenerated absorbent. a combustion apparatus, generating an exhaust gas, an air heater for recovering heat contained in the exhaust gas from the combustion apparatus and for applying the heat recovered by the air heater into combustion air to be supplied to the combustion apparatus, a first heat exchanger for recovering heat contained in exhaust gas from the air heater and for applying the heat recovered by the first heat exchanger into a heat medium, an electrostatic precipitator for electrostatic precipitation treatment of exhaust gas from the first heat exchanger, a wet desulfurization device for wet desulfurization of exhaust gas from the electrostatic precipitator, a pre-scrubber for pre-scrubbing exhaust gas from the wet desulfurization device, an absorption column for receiving exhaust gas from the pre-scrubber for bringing the exhaust gas into contact with an absorbent to obtain a CO 2 absorbed absorbent and a CO 2 removed gas, a desorption column for releasing CO 2 from the CO 2 absorbed absorbent, to obtain a regenerated absorbent in a liquid state, a reboiler for vaporizing regenerated absorbent in a liquid state to generate regenerated absorbent in a gas state which supplies heat to the desorption column, a flash tank for flash vaporizing the regenerated absorbent recovered in a liquid state from the reboiler, a vapor recompression apparatus for compressing vapor withdrawn from an upper part of the flash tank to be fed to the desorption column, and a second heat exchanger for transferring heat to the CO 2 absorbed absorbent from the regenerated absorbent withdrawn in the liquid state from the flash tank; the CO 2 absorbed absorbent being able to be supplied from the absorption column to the desorption column via the second heat exchanger, and the regenerated absorbent being able to be supplied from the desorption column to the absorption column via the flash tank and the second heat exchanger in this order, wherein the system further comprises a third heat exchanger provided between the second heat exchanger and the desorption column to apply the heat recovered by the heat medium to the CO 2 absorbed absorbent and/or a fourth heat exchanger provided between the flash tank and the second heat exchanger to apply the heat recovered by the heat medium to the regenerated absorbent.