A CO 2 recovery unit comprising: a CO 2 absorber that brings a gas to be treated and a CO 2 -absorbing solution into contact with each other to cause CO 2 included in the gas to be treated to be absorbed into the CO 2 -absorbing solution, and supplies the CO 2 -absorbing solution which has absorbed the CO 2 to a CO 2 -absorbing solution regenerator through a rich solution supply tube; the CO 2 -absorbing solution regenerator that heats the CO 2 -absorbing solution which has absorbed CO 2, releases CO 2 from the CO 2 -absorbing solution, regenerates the CO 2 -absorbing solution, and supplies the CO 2 -absorbing solution which has been regenerated to the CO 2 absorber through a lean solution supply tube; a first sensor that detects a CO 2 concentration in a flue gas exhausted from the CO 2 absorber; a second sensor that detects a gas flow rate and a concentration of a CO 2 gas exhausted from the CO 2 -absorbing solution regenerator; and at least one of: a CO 2 recovery rate control unit that, on the basis of an actual measured value and a target value of a recovery rate of CO 2 from the gas to be treated, changes a circulation amount of the CO 2 -absorbing solution to be supplied to the CO 2 absorber and changes a supply amount of saturated steam to be supplied to a regeneration heater of the CO 2 -absorbing solution regenerator to control a difference value between the actual measured value and the target value of the CO 2 recovery rate to be within a predetermined range; and a CO 2 recovery amount control unit that, on the basis of an actual measured value of CO2 recovery amount detected by the second sensor in the gas released to the outside from the CO2-absorbing solution regenerator and a target value of the CO 2 recovery amount, changes the circulation amount of the CO 2 -absorbing solution to be supplied to the CO 2 absorber and changes the supply amount of the saturated steam to be supplied to the regeneration heater of the CO 2 -absorbing solution regenerator to control a difference value between the actual measured value and the target value of the CO 2 recovery amount to be within a predetermined range. a CO 2 absorber that brings a gas to be treated and a CO 2 -absorbing solution into contact with each other to cause CO 2 included in the gas to be treated to be absorbed into the CO 2 -absorbing solution, and supplies the CO 2 -absorbing solution which has absorbed the CO 2 to a CO 2 -absorbing solution regenerator through a rich solution supply tube; the CO 2 -absorbing solution regenerator that heats the CO 2 -absorbing solution which has absorbed CO 2, releases CO 2 from the CO 2 -absorbing solution, regenerates the CO 2 -absorbing solution, and supplies the CO 2 -absorbing solution which has been regenerated to the CO 2 absorber through a lean solution supply tube; a first sensor that detects a CO 2 concentration in a flue gas exhausted from the CO 2 absorber; a second sensor that detects a gas flow rate and a concentration of a CO 2 gas exhausted from the CO 2 -absorbing solution regenerator; and at least one of: a CO 2 recovery rate control unit that, on the basis of an actual measured value and a target value of a recovery rate of CO 2 from the gas to be treated, changes a circulation amount of the CO 2 -absorbing solution to be supplied to the CO 2 absorber and changes a supply amount of saturated steam to be supplied to a regeneration heater of the CO 2 -absorbing solution regenerator to control a difference value between the actual measured value and the target value of the CO 2 recovery rate to be within a predetermined range; and a CO 2 recovery amount control unit that, on the basis of an actual measured value of CO2 recovery amount detected by the second sensor in the gas released to the outside from the CO2-absorbing solution regenerator and a target value of the CO 2 recovery amount, changes the circulation amount of the CO 2 -absorbing solution to be supplied to the CO 2 absorber and changes the supply amount of the saturated steam to be supplied to the regeneration heater of the CO 2 -absorbing solution regenerator to control a difference value between the actual measured value and the target value of the CO 2 recovery amount to be within a predetermined range. a CO 2 recovery rate control unit that, on the basis of an actual measured value and a target value of a recovery rate of CO 2 from the gas to be treated, changes a circulation amount of the CO 2 -absorbing solution to be supplied to the CO 2 absorber and changes a supply amount of saturated steam to be supplied to a regeneration heater of the CO 2 -absorbing solution regenerator to control a difference value between the actual measured value and the target value of the CO 2 recovery rate to be within a predetermined range; and a CO 2 recovery amount control unit that, on the basis of an actual measured value of CO2 recovery amount detected by the second sensor in the gas released to the outside from the CO2-absorbing solution regenerator and a target value of the CO 2 recovery amount, changes the circulation amount of the CO 2 -absorbing solution to be supplied to the CO 2 absorber and changes the supply amount of the saturated steam to be supplied to the regeneration heater of the CO 2 -absorbing solution regenerator to control a difference value between the actual measured value and the target value of the CO 2 recovery amount to be within a predetermined range.