Abstract: In an absorption tower of a wet lime-gypsum process desulfurization plant which desulfurizes an exhaust gas by bringing an incoming exhaust gas into contact with an absorbent liquid circulated therethrough by means of a plurality of pumps, an optimum pH value of the absorbent liquid and an optimum number of working pumps are set according to an amount of an exhaust gas load of the absorption tower, and a feed rate of the absorbent liquid and a number of working pumps are controlled on the basis of these set values. According to one mode of embodiment of the invention, a variation rate of the exhaust gas load and a desulfurization ratio are detected, and when the amount of exhaust gas load is constant or rising and also the desulfurization ratio is lower than a desired desulfurization ratio, the number of working pumps is correctively increased.