To provide a production method of an inexpensive and highly efficient catalyst for removing of NOx by reduction in an exhaust gas, to provide a method for reduction removing of NOx in the exhaust gas, and to provide a cleaning method of a sintering exhaust gas.

(1) After a steel pickling waste liquid containing a ferrous ion is neutralized with an alkaline agent, the ferrous ion in the waste liquid is oxidized to form a precipitate containing Fe(OH)3, or (2) after the ferrous ion in the waste liquid is oxidized and thereafter is neutralized with the alkaline agent to form the precipitate containing Fe(OH)3, and then the precipitate is dehydrated and thereafter is molded to produce the catalyst. In addition, the produced catalyst is brought into contact with the exhaust gas added with NH3 to remove NOx by reduction in the exhaust gas.