A operation control method of a CO2 recovering apparatus (10A, 10B, 10C) comprising: a cooling tower for allowing an exhaust gas containing CO2 to contact water so as to cool the exhaust gas; a CO2 absorber including a CO2 recovering unit that brings the exhaust gas into contact with a CO2 absorbing solution absorbing CO2 to eliminate CO2 contained the exhaust gas; and a regenerator that eliminates CO2 contained in the CO2 absorbing solution having absorbed CO2 to regenerate the CO2 absorbing solution, the operation control method comprising: recycling, by the CO2 absorber, the CO2 absorbing solution obtained by eliminating CO2 by the regenerator; measuring, by an absorbing solution level meter, a liquid level of the CO2 absorbing solution at a tower bottom of the CO2 absorber; and performing, by a level controller, one of automatic control and manual control for a supply amount of makeup water to be supplied to the CO2 absorber, based on the liquid level of the CO2 absorbing solution detected by the absorbing solution level meter, wherein the level controller is switched to the automatic control while a change in the gas supply amount of the exhaust gas to be supplied to the CO2 absorber is finished, and the level controller is switched to the manual control while the gas supply amount of the exhaust gas to be supplied to the CO2 absorber is changed, to thereby maintain, at a constant level, the supply amount of the makeup water, change the liquid level of the CO2 absorbing solution and maintain, at a constant level, a concentration of the CO2 absorbing solution. a cooling tower for allowing an exhaust gas containing CO2 to contact water so as to cool the exhaust gas; a CO2 absorber including a CO2 recovering unit that brings the exhaust gas into contact with a CO2 absorbing solution absorbing CO2 to eliminate CO2 contained the exhaust gas; and a regenerator that eliminates CO2 contained in the CO2 absorbing solution having absorbed CO2 to regenerate the CO2 absorbing solution, the operation control method comprising: recycling, by the CO2 absorber, the CO2 absorbing solution obtained by eliminating CO2 by the regenerator; measuring, by an absorbing solution level meter, a liquid level of the CO2 absorbing solution at a tower bottom of the CO2 absorber; and performing, by a level controller, one of automatic control and manual control for a supply amount of makeup water to be supplied to the CO2 absorber, based on the liquid level of the CO2 absorbing solution detected by the absorbing solution level meter, wherein the level controller is switched to the automatic control while a change in the gas supply amount of the exhaust gas to be supplied to the CO2 absorber is finished, and the level controller is switched to the manual control while the gas supply amount of the exhaust gas to be supplied to the CO2 absorber is changed, to thereby maintain, at a constant level, the supply amount of the makeup water, change the liquid level of the CO2 absorbing solution and maintain, at a constant level, a concentration of the CO2 absorbing solution. recycling, by the CO2 absorber, the CO2 absorbing solution obtained by eliminating CO2 by the regenerator; measuring, by an absorbing solution level meter, a liquid level of the CO2 absorbing solution at a tower bottom of the CO2 absorber; and performing, by a level controller, one of automatic control and manual control for a supply amount of makeup water to be supplied to the CO2 absorber, based on the liquid level of the CO2 absorbing solution detected by the absorbing solution level meter, wherein the level controller is switched to the automatic control while a change in the gas supply amount of the exhaust gas to be supplied to the CO2 absorber is finished, and the level controller is switched to the manual control while the gas supply amount of the exhaust gas to be supplied to the CO2 absorber is changed, to thereby maintain, at a constant level, the supply amount of the makeup water, change the liquid level of the CO2 absorbing solution and maintain, at a constant level, a concentration of the CO2 absorbing solution.