An absorbent regenerator 15 is divided into at least two parts and includes: a rich solution supply line L 1 through which a rich solution 14 is supplied to the absorbent regenerator 15 from a CO 2 absorber 13; a lean solution supply line L 2 through which a lean solution 16 is supplied to the CO 2 absorber 13 from the absorbent regenerator 15; a lean-rich solution heat exchanger 17 that is provided at an intersection of the lean solution supply line L 2 and the rich solution supply line L 1 to exchange heat between the lean solution 16 and the rich solution 14; a branch portion 18 that branches some 14a of the rich solution 14 at a downstream side of the lean-rich solution heat exchanger 17 on the rich solution supply line L 1 ; and a first mixing portion 20a that mixes the some 14a of the rich solution 14 branched at the branch portion 18 with a semi-lean solution 19 in which CO 2 has been partially removed from the rich solution 14 in the absorbent regenerator 15.