To provide a unit which preliminarily filters out dust particles in a harmful gas-containing gaseous discharge before gas filtration.

The new convenient catalytic conversion unit 68 for preliminarily filtering out dust particles from a gaseous discharge containing an unfavorable gaseous component, e.g. an ozone-containing gaseous discharge from an electrostatic copying machine before passing through a gas neutralization catalyst bed has a dust filter screen 88 which can easily be detached, washed and/or exchanged. The filter screen 88 has a dust collecting mesh disposed around and near the catalyst layer or bed and the screen 88 is supported in such a way that it can easily be detached from the conversion unit 68 without causing interference with the catalyst layer or bed so as to carry out periodic washing or exchange.