PURPOSE: To attain the improvement of dust collecting efficiency, the prevention of pressure loss and the improvement of economization, etc., to also attain a sufficient application to existing equipment, to efficiently remove nitrogen oxide from waste gas, and to also easily make a bag filter.

CONSTITUTION: A reducing agent for nitrogen oxide and a reducing catalyst are injected from a reducing agent jetting device 4 and a reducing catalyst jetting device 5 to waste gas on the upstream side of a bag filter installed in a waste gas filter 6 to reduce the nitrogen oxide in the waste gas until the reducing catalyst reaches filter cloth of the bag filter, and the reducing catalyst is collected by the bag filter. The nitrogen oxide in the waste gas which has not been reduced before it reaches the filter cloth is reduced by the catalyst collected by the filter cloth and discharged in the air. Thus, nitrogen oxide is removed from waste gas by using filter cloth currently in use without a special treatment being newly made on the filter cloth.