PURPOSE: To prevent the generation of inconveniences which permit the penetration of various germs from outside into a working room by installing a pre-filter on the downstream side of a suction port in the rear of said working room in an exhaust duct.

CONSTITUTION: A pre-filter 19, which is installed in a slanting manner on the downstream side of a suction port 7 in the rear of a working room 1 in an exhaust duct 8, is designed to filter collectively coarse dust contained in the air which flows into the exhaust duct 8 from suction ports 6 and 7. Furthermore, an inspection cover 20, which is removable, is installed to the rear wall of the working room 1 by way of a joint section 21. The inspection cover 20 is provided with an inspection window 22 which can help examine visually how clogged the pre-filter is with the dust. Since the pre-filter is installed to the suction ports 6 and 7, there is no movement on an air current branch surface inside the working room 1. It is, therefore, possible to eliminate the contamination produced by the penetration of various germs into the working room 1 from the outside accompanying fluctuations in the air current inside the working room 1. It is also possible to disinfect by spraying disinfectant over the densely contaminated side of the pre-filter 19.