A CO 2 recovery system comprising: a CO 2 absorber for bringing a cooled CO 2 -containing exhaust gas into contact with a CO 2 -absorbent absorbing CO 2 so as to remove CO 2 from the exhaust gas ; a first absorbent regenerator (18-1) for regenerating an absorbent by releasing CO 2 from a CO 2 -absorbent having absorbed CO 2 ; a second absorbent regenerator (18-2) for regenerating an absorbent by releasing residual CO 2 from a first lean solution discharged from the first absorbent regenerator (18-1) through a first lean solution supply line (L3); and a regeneration superheater for superheating a second lean solution ; a flash drum for flashing the second lean solution discharged from the second absorbent regenerator (18-2), wherein vapor generated in the flash drum is inputted to the first absorbent regenerator (18-1), and wherein the CO 2 recovery system further comprises: a port (A) provided in the first absorbent regenerator (18-1) at a middle portion in a vertical axis direction of the first absorbent regenerator (18-1), for extracting a semi-lean solution ; a circulation line (L 7 ) to which the semi-lean solution is extracted, for returning the semi-lean solution to a return port (B) closer to the bottom than the extraction port (A); and a semi-lean/lean solution heat exchanger provided on the circulation line (L 7 ), for heat-exchanging a third lean solution flashed in the flash drum with the semi-lean solution . a CO 2 absorber for bringing a cooled CO 2 -containing exhaust gas into contact with a CO 2 -absorbent absorbing CO 2 so as to remove CO 2 from the exhaust gas ; a first absorbent regenerator (18-1) for regenerating an absorbent by releasing CO 2 from a CO 2 -absorbent having absorbed CO 2 ; a second absorbent regenerator (18-2) for regenerating an absorbent by releasing residual CO 2 from a first lean solution discharged from the first absorbent regenerator (18-1) through a first lean solution supply line (L3); and a regeneration superheater for superheating a second lean solution ; a flash drum for flashing the second lean solution discharged from the second absorbent regenerator (18-2), wherein vapor generated in the flash drum is inputted to the first absorbent regenerator (18-1), and wherein the CO 2 recovery system further comprises: a port (A) provided in the first absorbent regenerator (18-1) at a middle portion in a vertical axis direction of the first absorbent regenerator (18-1), for extracting a semi-lean solution ; a circulation line (L 7 ) to which the semi-lean solution is extracted, for returning the semi-lean solution to a return port (B) closer to the bottom than the extraction port (A); and a semi-lean/lean solution heat exchanger provided on the circulation line (L 7 ), for heat-exchanging a third lean solution flashed in the flash drum with the semi-lean solution . a port (A) provided in the first absorbent regenerator (18-1) at a middle portion in a vertical axis direction of the first absorbent regenerator (18-1), for extracting a semi-lean solution ; a circulation line (L 7 ) to which the semi-lean solution is extracted, for returning the semi-lean solution to a return port (B) closer to the bottom than the extraction port (A); and a semi-lean/lean solution heat exchanger provided on the circulation line (L 7 ), for heat-exchanging a third lean solution flashed in the flash drum with the semi-lean solution . <EOS>