A combustion exhaust gas treatment system comprising: a heat exchanger (A) for recovering heat contained in the combustion exhaust gas into heat medium, an absorption column for obtaining CO2 removed gas by absorbing CO2 in the combustion exhaust gas into absorbent, a heat exchanger (B) for applying heat recovered by heat medium to the CO2 absorbed absorbent, a desorption column for desorbing the absorbent by removing CO2 from the CO2 absorbed absorbent, a flash tank for flash vaporizing the desorbed absorbent and a heat exchanger (E) for transferring heat from the desorbed absorbent to the CO2 absorbed absorbent, wherein the CO2 absorbed absorbent can be supplied from the absorption column to the desorption column via the heat exchanger (E) and the heat exchanger (B) in this order, and the desorbed absorbent can be supplied from the desorption column to the absorption column via the flash tank and the heat exchanger (E) in this order.