A CO 2 recovery system (10C, 10D) 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 for regenerating an absorbent by releasing CO 2 from a CO 2 -absorbent having absorbed CO 2; a second absorbent regenerator for regenerating an absorbent by releasing residual CO 2 from a first lean solution discharged from the first absorbent regenerator through a first lean solution supply line; 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, wherein vapor generated in the flash drum is inputted to the first absorbent regenerator, and wherein the CO 2 recovery system (10C, 10D) further comprises: a port provided in the first absorbent regenerator at a middle portion in a vertical axis direction of the first absorbent regenerator, 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 closer to the bottom than the extraction port; 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 for regenerating an absorbent by releasing CO 2 from a CO 2 -absorbent having absorbed CO 2; a second absorbent regenerator for regenerating an absorbent by releasing residual CO 2 from a first lean solution discharged from the first absorbent regenerator through a first lean solution supply line; 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, wherein vapor generated in the flash drum is inputted to the first absorbent regenerator, and wherein the CO 2 recovery system (10C, 10D) further comprises: a port provided in the first absorbent regenerator at a middle portion in a vertical axis direction of the first absorbent regenerator, 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 closer to the bottom than the extraction port; 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 provided in the first absorbent regenerator at a middle portion in a vertical axis direction of the first absorbent regenerator, 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 closer to the bottom than the extraction port; 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.