PURPOSE: To decrease the consumption of a desulfurizing agent and to eliminate the necessity of instruments for detecting impurities in a desulfurizing agent slurry.

CONSTITUTION: On the basis of the gas properties A1 at the inlet of an exhaust gas 2 flowing in a gas introduction system 103 installed on the input side of a desulfurizer 101, the gas properties A2 at the outlet of an exhaust gas 5 flowing in a gas discharge system 104 installed on the output side of the desulfurizer 101, and a factor Z affecting the degree of desulfurization of the desulfurizer 101, a message B for correcting the factor affecting the degree of desulfuration is obtained in a fuzzy control system 102. As the result of the correction of the factor Z affecting the degree of desulfuration of the desulfurizer 101 by the message B for correcting the factor affecting the degree of desulfuration, the consumption of the desulfurizing agent is decreased and the detector for detecting impurities in a desulfurizing agent slurry becomes needless.