PURPOSE: To operate the absorption tower with less amount of absorbent by adjusting the amount of absorbent to be supplied to the absorption tower by changing a ratio set value with the control deviation sent from a function generating unit based on the deviation of carbonate concentration.

CONSTITUTION: The concn. of carbonate in liq. absorbent is controlled by adjusting the amount of absorbent to be supplied to the adsorption tower 1, and the amount of absorbent is obtained in such a way that the flow rate of SO2 is obtained from the flow rate of the exhaust gas entered into the absorption tower 1 and the concn. of SO2, and the obtained flow rate of SO2 is multiplied by the ratio set value sent from a ratio setting unit 19. The concn. of carbonate in the liq. absorbent in the absorption tower 1 is detected to obtain the deviation from the set value of the concn. of carbonate. By the control deviation outputted from the function generating unit 18 based on the deviation of the concn. of carbonate, the set value of the ratio is changed and the absorbent amount to be supplied to the absorption tower 1 is adjusted. As a result, the error of the supplied amount of absorbent based on the errors of a flow meter 11 of exhaust gas and a concn. meter 12 of SO2, etc., are corrected so that the optimum control of the concn. of carbonate is carried out to operate with a small amount of absorbent.