A method for treating an ammonia (NH.sub.3)-containing gas, which comprises causing the NH.sub.3-containing gas to pass through a catalyst tower (9) firstly with a pre-treatment catalyst layer (1) having, in parallel, a flow path involving a catalyst layer having the function of oxidizing NH.sub.3 to form NO and a flow path involving a catalyst layer not having the above function, and then, contacting the resultant gas with a catalyst layer (2) having in combination, the denitration function and the function of oxidizing NH.sub.3 to form NO; and an apparatus for practicing the method.