A composite catalyst which is capable of clarifying exhaust gas containing nitrogen oxides therein with a high effectiveness over a long period of time while producing no or a very small amount of undesirable by-products such as ammonia, which comprises a catalytic ingredient comprising oxides of nickel and copper in an atomic ratio of nickel to copper of 1:0.05 through 1 and a catalytic base comprising a refractory inorganic material containing at least 50% by weight of zircon therein, said catalytic ingredient being carried on said catalytic base.