PURPOSE: To reduce the oxidative conversion of SO2 to SO3 in a process of the catalytic reduction of nitrogen oxides in a waste gas with the aid of ammonia in the presence of a catalyst comprising promarily titanium dioxide by adding an alcohol to the waste gas prior, after that heating and passing the waste gas through the catalyst.

CONSTITUTION: A process for removing nitrogen oxides from a waste gas by catalytic reduction of nitrogen oxides to form nitrogen with the aid of ammonia in the presence of a catalyst containing titanium dioxide as a primary component and vanadium and/or tungsten or the like, reduces the oxidative conversion of SO2 to SO3 and does not generally spoil the catalytic reductive conversion of nitrogen oxides by adding 1-10C aliphatic alcohol such as butanol and alicyclic or aromatic alcohol to the waste gas prior to passing the waste gas through the catalyst and then heating the waste gas at 150-550°C.