Provided are a CO2 absorber that reduces CO2 contained in flue gas; a regenerator that reduces CO2 contained in rich solvent absorbing CO2 to regenerate the rich solvent, so that lean solvent having the CO2 reduced in the regenerator is reused in the CO2 absorber; a heat exchanger that allows the rich solvent to exchange heat with the lean solvent; and a controller that controls to extract rich solvent portion that is part of the rich solvent, to allow the rich solvent portion to by pass the heat exchanger, and to be supplied into the top of the regenerator without exchanging heat so as to minimize a sum of an enthalpy that is taken out of the regenerator as CO2 gas accompanying steam and an enthalpy of the lean solvent after heat exchange with the rich solvent in the heat exchanger.