A catalyst for denitrizing nitrogen oxides contained in waste gases, which comprises:(a) titanium, and(b) at least one base metal selected from the group consisting of V, W, Mo, Mn, Cu, Fe, Cr, Co, Ni, Zn and Sn, the catalyst containing titanium and the base metals in amounts of at least about 50% by weight in terms of oxides based on the catalyst, and titanium in amounts of about 80-99% by weight in terms of oxides and the base metals in amounts of about 20-1% by weight in terms of oxides, respectively, based on the total weight in terms of oxides of titanium and the base metals, and the catalyst having micropores of about 50-75000 .ANG. in radius in amounts of about 0.25-0.45 ml/g, and micropores of about 50-100 .ANG. in radius in amounts of about 10-40% by volume and micropores of about 500-60000 .ANG. in radius in amounts of not less than about 10% by volume, respectively, based on the total volume of the micropores of about 50-75000 .ANG. in radius, in the presence of a reducing gas at elevated temperatures.The catalyst retains a higher denitrizing activity over a long period time than the conventional catalysts, and is substantially free from deactivation with arsenic or arsenic compounds contained in waste gases.