Abstract: In a system wherein exhaust gases containing SO.sub.2 are fed to a sulfuric acid plant from multiple furnaces, including an intermittently operated furnace (such as a copper or nickel converter furnace), a method of automatically controlling the gas pressure to within a predetermined range at a point just upstream of the exhaust gas pretreating equipment at the inlet of the sulfuric acid plant so as to compensate for the fluctuation in the exhaust gas pressure resulting from start up and stopping of the operation of the intermittently-operated furnace. By this method, the disadvantages caused by large gas fluctuations, i.e., the leakage of SO.sub.2 -containing gas and the suction of excess free air can be prevented.