Nitrogen oxides contained in exhaust gas from lean-burn engines-mounted internal combustion engines are effectively controlled. An exhaust emission control system and method therefor for controlling nitrogen oxides contained in combustion exhaust gas from internal combustion engines in the presence of a catalyst using reducing gases such as carbon monoxide and hydrogen carbide contained in the exhaust gas, wherein the catalyst used consists of a porous carrier, all of Rh, Pt and Pd, at least selected one of alkali metal and alkali earth metal and Mn or its compound, all formed on the porous carrier.