For a system having a source of nitrogen oxide-containing gas, an ammonia reduction denitrator, and a leak ammonia recovery unit which adsorbs and removes ammonia escaping from said ammonia reduction denitrator, a method of removing adsorbed ammonia from said leak ammonia recovery unit and reusing the removed ammonia is provided which includes the steps of: introducing hot exhaust gas discharged from said nitrogen oxide-containing gas source into said leak ammonia recovery unit to desorb ammonia therefrom; after desorption of ammonia, cooling said heated leak ammonia recovery unit with cool air; and feeding ammonia carried away by said hot exhaust gas and said cool air to an upstream side of said ammonia reduction denitrator or to the source of nitrogen oxide-containing gas.