Apparatus for treating exhaust gases comprising a catalyst bed through which the exhaust gas is passed whereby nitrogen oxides and dust present therein are removed from the gas, said catalyst bed being designed such that the catalyst therein is discharged from the bed continuously or intermittantly while being fed in an amount that can compensate for the discharged amount, a heating device for heating the catalyst discharged from the bed to a temperature in a range from about 700.degree. to 1200.degree. C. for reactivation of the catalyst and separation of the dust accumulated on the bed surface and retained in the interstices between the catalyst particles therein, and a device for discharging sulfur dioxides produced during the course of heating the catalyst. This apparatus is very effective in simultaneously removing the nitrogen oxides and the dust from exhaust gases such as boiler exhaust gases.