A CO 2 recovery device comprising: a CO 2 absorption tower that brings an exhaust gas containing CO 2 arriving from a generation source of the exhaust gas into contact with an absorbent to eliminate CO 2 in the exhaust gas; and a regeneration tower that eliminates and regenerates CO 2 in a rich solution that has absorbed CO 2 in the CO 2 absorption tower, wherein the absorbent, which is a lean solution from which CO 2 is eliminated in the regeneration tower, is reused in the CO 2 absorption tower, a first flow channel that connects the generation source and the absorption tower; a main valve that adjusts the volume of the exhaust gas G1 to be supplied to the absorption tower is provided on the first flow channel; a second flow channel that branches from the first flow channel; a branch valve is provided which on the second flow channel and a tip of the second flow channel) is connected to a chimney; the branch valve adjusting the volume of discharge of the exhaust gas from the chimney; a CO 2 concentration sensor that measures the concentration Y CO2 of C0 2 contained in the exhaust gas; a computing unit, and a driving control unit; a setting unit that sets operation parameters; wherein an exhaust gas treatment ratio R G1 and a CO 2 recovery ratio R CO2 shown below are set as operation parameters of the CO 2 recovery device, wherein a target treatment volume for exhaust gas to be treated by the CO 2 recovery device is specified on the basis of the product of an actual flow rate V G1 of the exhaust gas discharged toward the CO 2 recovery device from the generation source of the exhaust gas, and the exhaust gas treatment ratio R G1 set as operational parameter the driving control unit controls the opening degrees of the main valve and the branch valve on the basis of the setting value; and wherein a target recovery volume for CO 2 to be recovered by the CO 2 recovery device is specified on the basis of the product of the concentration Y CO2 of C0 2 contained in the exhaust gas discharged from the generation source of the exhaust gas to the CO 2 recovery device, the measurement value V G1 acquired from the exhaust gas flow rate sensor, the exhaust gas treatment ratio R G1, and the CO 2 recovery ratio R CO2, wherein. R G 1 = T CO 2 / V G 1 T CO2: Volume of exhaust gas to be treated by CO 2 recovery device; V G1: Flow rate of exhaust gas discharged from generation source of exhaust gas R CO 2 = C CO 2 / V CO 2 C CO2: Volume of CO 2 to be recovered by CO 2 recovery device; V CO2: Volume of CO 2 contained in exhaust gas to be treated by CO 2 recovery device a CO 2 absorption tower that brings an exhaust gas containing CO 2 arriving from a generation source of the exhaust gas into contact with an absorbent to eliminate CO 2 in the exhaust gas; and a regeneration tower that eliminates and regenerates CO 2 in a rich solution that has absorbed CO 2 in the CO 2 absorption tower, wherein the absorbent, which is a lean solution from which CO 2 is eliminated in the regeneration tower, is reused in the CO 2 absorption tower, a first flow channel that connects the generation source and the absorption tower; a main valve that adjusts the volume of the exhaust gas G1 to be supplied to the absorption tower is provided on the first flow channel; a second flow channel that branches from the first flow channel; a branch valve is provided which on the second flow channel and a tip of the second flow channel) is connected to a chimney; the branch valve adjusting the volume of discharge of the exhaust gas from the chimney; a CO 2 concentration sensor that measures the concentration Y CO2 of C0 2 contained in the exhaust gas; a computing unit, and a driving control unit; a setting unit that sets operation parameters; wherein an exhaust gas treatment ratio R G1 and a CO 2 recovery ratio R CO2 shown below are set as operation parameters of the CO 2 recovery device, wherein a target treatment volume for exhaust gas to be treated by the CO 2 recovery device is specified on the basis of the product of an actual flow rate V G1 of the exhaust gas discharged toward the CO 2 recovery device from the generation source of the exhaust gas, and the exhaust gas treatment ratio R G1 set as operational parameter the driving control unit controls the opening degrees of the main valve and the branch valve on the basis of the setting value; and wherein a target recovery volume for CO 2 to be recovered by the CO 2 recovery device is specified on the basis of the product of the concentration Y CO2 of C0 2 contained in the exhaust gas discharged from the generation source of the exhaust gas to the CO 2 recovery device, the measurement value V G1 acquired from the exhaust gas flow rate sensor, the exhaust gas treatment ratio R G1, and the CO 2 recovery ratio R CO2, wherein. R G 1 = T CO 2 / V G 1 T CO2: Volume of exhaust gas to be treated by CO 2 recovery device; V G1: Flow rate of exhaust gas discharged from generation source of exhaust gas R CO 2 = C CO 2 / V CO 2 C CO2: Volume of CO 2 to be recovered by CO 2 recovery device; V CO2: Volume of CO 2 contained in exhaust gas to be treated by CO 2 recovery device T CO2: Volume of exhaust gas to be treated by CO 2 recovery device; V G1: Flow rate of exhaust gas discharged from generation source of exhaust gas C CO2: Volume of CO 2 to be recovered by CO 2 recovery device; V CO2: Volume of CO 2 contained in exhaust gas to be treated by CO 2 recovery device