PURPOSE: To enlarge the area of the filter of a fan filter unit as for as possible, and to supply clear air by splitting or sliding a filter casing, shrinking the filter casing and thereafter housing the same into a ceiling, reexpanding it in the ceiling and assembling it.

CONSTITUTION: The area of a filter is made as maximum as possible by splitting or sliding a filter casing 12, shrinking the filter casing 12, housing the same into a ceiling 20, expanding it in the ceiling and assembling it. As a result, the area of the filter in a fan filter unit is used as maximumly as possible and the filter is effectively utilized. Clean air is efficiently obtained. Further the stagnation region of dust is narrowed and also caulking seal is performed at a stroke.