To obtain a high face air velocity without lowering the supplementary efficiency of a filter, to remove thereby contaminants quickly and to supply clean air with force to a distance far from a filter face.

An adaptor 1c is formed in the shape of a funnel-like restrictor toward an outlet opening face 1d in the lower part, while the lower part is shaped like a truncated pyramid. The lower end part of the adaptor 1c is formed to be a vertical guide part 1e which is linear on a longitudinal section. The central axis of the adaptor 1c coincides with the central axes of a unit main body 1a and an air filter 1b. Since the quantity of air per a unit area let out from the outlet opening face 1d increases, the velocity of air becomes higher at the outlet opening face 1d than that on the occasion of passing through the air filter 1b.