The following devices are successively disposed in the following order from an upstream side to a downstream side in an exhaust gas duct of a combustion apparatus: an air preheater, preheating combustion air for use in an exhaust gas treating apparatus; a heat recovery unit, recovering exhaust gas heat at an exit of the air preheater; a precipitator, collecting soot/dust contained in an exhaust gas at an exit of the heat recovery unit; a wet flue gas desulfurizer, removing sulfur oxides contained in the exhaust gas at the exit of the precipitator; and a reheater, heating the exhaust gas at the exit of the wet flue gas desulfurizer. Each of the heat recovery unit and the reheater has a heat exchanger tube, and a circulation line is disposed to connect the heat exchanger tubes. A sulfur trioxide (SO.sub.3) removing agent is supplied to the upstream side of the heat recovery unit, and the temperature of the exhaust gas at the exit of the heat recovery unit is adjusted to not more than a dew point of sulfur trioxide. As the sulfur trioxide removing agent, use is preferably made of at least one among a sulfur trioxide adsorbent, a sulfur trioxide reducing agent, and a sulfur trioxide neutralizing agent. Thus, even when coal with a high sulfur content is used as fuel, heavy metals contained in the exhaust gas can be removed effectively from the exhaust gas.