PURPOSE: To carry out a dust removal and a denitration collectively without need of a wide installing area by attaching a bag filter to a honeycomb style supportor supporting a denitration catalyst and treating a waste gas to be treated which is formed by mixing ammonia into a waste gas contg. soot and dust and nitrogen oxides.

CONSTITUTION: A bag filter 2 consisting of a wavon fabric or non-wavon fabric of fiber is attached to an inside or an outside honeycomb style supporter 1 made of titanium oxide, zeolite, etc., supporting a denitration catalist made of gold, platinum, etc. By using the filter for the dust removal and the denitration, waste gas contg. the soot and dust and the nitrogen oxides is further injected with ammonia for a denitration and is treated. Thus, the dust removal and the denitration are collectively carried out and lowering of a denitration efficiency caused by attachment and sedimentation of the soot and the dust is prevented, and a soot and a dust attached to the filter are cleaned and removed.