A CO 2 recovery device (10A; 10B; 10C), comprising: a CO 2 absorber including a CO 2 absorbing section in which a flue gas and a basic amine compound absorbent are brought into contact with each other to allow CO 2 in the flue gas to be absorbed in the basic amine compound absorbent, and an aqueous cleaning section in which a decarbonated flue gas, from which CO 2 is already removed in the CO 2 absorbing section, and rinsing water are brought into contact with each other to remove an accompanying substance that accompanies the decarbonated flue gas, and an absorbent regenerator that separates CO 2 from a rich solution to which CO 2 is already absorbed, and regenerates the basic amine compound absorbent into a lean solution, wherein the lean solution from which CO 2 is already removed is re-used as the basic amine compound absorbent in the CO 2 absorber; a degassing basin that is interposed in a rich solution supply line (L 11) that supplies the rich solution from the CO 2 absorber to the absorbent regenerator (14 wherein the degassing basin includes: a retaining section that retains the rich solution for a predetermined time and is configured to separate oxygen in the rich solution by floating separation, a partition wall that allows the rich solution, from which oxygen is flotation-separated in the retaining section, to gravity fall along a wall surface thereof and it is configured to remove oxygen, and a storage section that stores the rich solution which falls along the partition wall and from which oxygen is removed; and a purge gas introducing unit that purges a gas, which does not contain oxygen, into the degassing basin. a CO 2 absorber including a CO 2 absorbing section in which a flue gas and a basic amine compound absorbent are brought into contact with each other to allow CO 2 in the flue gas to be absorbed in the basic amine compound absorbent, and an aqueous cleaning section in which a decarbonated flue gas, from which CO 2 is already removed in the CO 2 absorbing section, and rinsing water are brought into contact with each other to remove an accompanying substance that accompanies the decarbonated flue gas, and an absorbent regenerator that separates CO 2 from a rich solution to which CO 2 is already absorbed, and regenerates the basic amine compound absorbent into a lean solution, wherein the lean solution from which CO 2 is already removed is re-used as the basic amine compound absorbent in the CO 2 absorber; a degassing basin that is interposed in a rich solution supply line (L 11) that supplies the rich solution from the CO 2 absorber to the absorbent regenerator (14 wherein the degassing basin includes: a retaining section that retains the rich solution for a predetermined time and is configured to separate oxygen in the rich solution by floating separation, a partition wall that allows the rich solution, from which oxygen is flotation-separated in the retaining section, to gravity fall along a wall surface thereof and it is configured to remove oxygen, and a storage section that stores the rich solution which falls along the partition wall and from which oxygen is removed; and a purge gas introducing unit that purges a gas, which does not contain oxygen, into the degassing basin. a retaining section that retains the rich solution for a predetermined time and is configured to separate oxygen in the rich solution by floating separation, a partition wall that allows the rich solution, from which oxygen is flotation-separated in the retaining section, to gravity fall along a wall surface thereof and it is configured to remove oxygen, and a storage section that stores the rich solution which falls along the partition wall and from which oxygen is removed; and a purge gas introducing unit that purges a gas, which does not contain oxygen, into the degassing basin.