This invention relates to a technique for removing nitrogen oxides (NOx) present in exhaust gases discharged from boilers and the like. When the temperature of the exhaust gas is 100° C. or below, a heat-treated active carbon produced by heat-treating a raw active carbon at 600 to 1,200° C. in a non-oxidizing atmosphere so as to remove oxygen-containing functional groups present at the surfaces thereof and thereby reduce the atomic surface oxygen/surface carbon ratio to 0.05 or less is preferably used. When the temperature of the exhaust gas exceeds 100° C., a heat-treated active carbon produced by heat-treating a raw active carbon at 600 to 1,200° C. in a non-oxidizing atmosphere and activating the surfaces thereof with sulfuric acid or nitric acid to impart oxidizing oxygen-containing functional groups thereto is preferably used.