To prevent dirt not only from entering a room but also to remove toxic substances therefrom by creating a pressurized blockade region under pressure higher than that outside the room just behind the entrance thereof.

Air, to be passed through a filter, is filtered by plates to create a region, which is higher in pressure than a region under the plates, beneath a gap defined by the edges of the plates and an inner surface 13 and in front of a working region 70. High pressure created a blockade region between the most of the room and outside air. As a result, a part of filtered air flows out from the blockade region via an opening 15 to prevent particles from entering the room, whereas toxic substances generated in the room are removed through discharge holes 44, a discharge plenum 42 and a discharge section 20, which are provided for treating. As a result, it is possible to create a clean working region and prevent the environment from being polluted due to hazardous dangerous materials used in the region.