A recovery unit (12A, 12B) of CO 2, H 2 S, or both of CO 2 and H 2 S, the recovery unit (12A, 12B) comprising: an absorber configured to bring a gas containing CO 2, H 2 S, or both of CO 2 and H 2 S, and an absorbent in contact to remove CO 2, H 2 S, or both of CO 2 and H 2 S; an absorbent regenerator configured to regenerate a rich solution that has absorbed CO 2, H 2 S, or both of CO 2 and H 2 S to obtain a lean solution, wherein the recovery unit (12A, 12B) is configured so that the rich solution is supplied from the absorber to the absorbent regenerator, and the lean solution obtained in the absorbent regenerator is supplied to the absorber to be reused, a CO 2 -entraining gas accompanied by water vapor discharged from the absorbent regenerator is condensed by a condenser and then condensed water is separated in a separation drum, and resulting reflux water is supplied to an upper portion of the absorbent regenerator and to a top of the absorber; a reclaimer configured to extract a part of the lean solution regenerated in the absorbent regenerator through an introduction line (L 11) to remove a coexisting substance in the lean solution; a heating section configured to heat the lean solution contained in the reclaimer to obtain recovered vapor; and a mixing tank disposed on the introduction line (L 11) and configured to receive the part of the lean solution and an alkaline agent for mixing thereof; a discharge line (L 12) through which the recovered vapor discharged from the reclaimer is introduced into the bottom of the absorbent regenerator; the recovery unit further comprising a gas-liquid separator provided in the discharge line (L 12), and configured to separate a coexisting substance entrained in the recovered vapor; a first cooler provided in the discharge line (L 12), and configured to cool the recovered vapor to be introduced into the gas-liquid separator; an introduction line (L 13) as part of the discharge line (L 12) through which a separated absorbent vapor from the gas liquid separator is supplied to the absorbent regenerator; a further discharge line (L 14) through which a liquid coexisting substance from the gas-liquid separator is discharged; and a cooling water line (L 15) through which a part of the resulting reflux water from the separation drum is introduced into the first cooler to cool the recovered vapor and is introduced into the mixing tank to dilute and mix the lean solution and the alkaline agent, to keep the uniform concentration distribution of each component. an absorber configured to bring a gas containing CO 2, H 2 S, or both of CO 2 and H 2 S, and an absorbent in contact to remove CO 2, H 2 S, or both of CO 2 and H 2 S; an absorbent regenerator configured to regenerate a rich solution that has absorbed CO 2, H 2 S, or both of CO 2 and H 2 S to obtain a lean solution, wherein the recovery unit (12A, 12B) is configured so that the rich solution is supplied from the absorber to the absorbent regenerator, and the lean solution obtained in the absorbent regenerator is supplied to the absorber to be reused, a CO 2 -entraining gas accompanied by water vapor discharged from the absorbent regenerator is condensed by a condenser and then condensed water is separated in a separation drum, and resulting reflux water is supplied to an upper portion of the absorbent regenerator and to a top of the absorber; a reclaimer configured to extract a part of the lean solution regenerated in the absorbent regenerator through an introduction line (L 11) to remove a coexisting substance in the lean solution; a heating section configured to heat the lean solution contained in the reclaimer to obtain recovered vapor; and a mixing tank disposed on the introduction line (L 11) and configured to receive the part of the lean solution and an alkaline agent for mixing thereof; a discharge line (L 12) through which the recovered vapor discharged from the reclaimer is introduced into the bottom of the absorbent regenerator; the recovery unit further comprising a gas-liquid separator provided in the discharge line (L 12), and configured to separate a coexisting substance entrained in the recovered vapor; a first cooler provided in the discharge line (L 12), and configured to cool the recovered vapor to be introduced into the gas-liquid separator; an introduction line (L 13) as part of the discharge line (L 12) through which a separated absorbent vapor from the gas liquid separator is supplied to the absorbent regenerator; a further discharge line (L 14) through which a liquid coexisting substance from the gas-liquid separator is discharged; and a cooling water line (L 15) through which a part of the resulting reflux water from the separation drum is introduced into the first cooler to cool the recovered vapor and is introduced into the mixing tank to dilute and mix the lean solution and the alkaline agent, to keep the uniform concentration distribution of each component.