A catalyst for denitrizing nitrogen oxides contained in waste gases in the presence of arsenic compounds, which comprises: titanium and vanadium therein in weight ratios of oxides of titanium to oxides of vanadium ranging from 99.9:0.1 to 92.0:8.0, the vanadium being concentratedly contained in the surface layer of the catalyst up to 200 .mu.m in depth from the surface of the catalyst in concentrations of at least about 1.5 times as much as the concentrations of vanadium throughout the catalyst.The catalyst may further contain therein at least one of tungsten and zirconium.The catalyst preferably has micropores of about not less than about 50 .ANG. in amounts of 0.25-0.40 ml/g, and micropores of about 50-100 .ANG. in radius in amounts of about 10-40% by volume and micropores of about 1000-60000 .ANG. in radius in amounts of not less than about 10% by volume, respectively, based on the total volume of the micropores of not less than about 50.ANG. in radius.The catalyst retains a higher denitrizing activity over a long period of denitrizing operations in the presence of arsenic compounds contained waste gases than the conventional catalysts.