A system is provided that prevents inhibition of adsorption of Hg and other heavy metals by activated carbon or other heavy metal adsorbent due to prior adsorption of sulfur trioxide (SO3) in an exhaust gas containing SO3. As it has been found that while SO3 is adsorbed, the adsorption of SO3 precedes the adsorption of Hg and other heavy metals onto activated carbon, a basic substance injection system is disposed along an exhaust gas flow channel at an upstream side of an activated carbon injection system, thereby attaining effective removal of Hg and other heavy metals from the exhaust gas by adsorption thereof onto surface pores of the activated carbon. The SO3 concentration after removal by basic substance conversion is computed from the SO3 concentration before removal, and the activated carbon injection rate can be controlled based on the concentration after removal.