The present invention provides: an exhaust gas treatment method by which treatment costs are lower than in the past, which efficiently reduces the concentration of SO[3] in a combustion exhaust gas, and which reduces the quantity of NOX; and a denitrification/SO[3] reduction apparatus. This exhaust gas treatment method comprises adding a compound that contains H element and C element as a first additive to a combustion exhaust gas that contains SO[3] in addition to NOX in an oxidizing atmosphere, and then bringing the combustion exhaust gas into contact with a catalyst that contains, as a carrier, oxides of at least one element selected from among the group consisting of Ti, Si, Zr and Ce and/or a mixed oxide and/or composite oxide of two or more elements selected from among this group, thereby reducing SO[3] to SO[2].