To avoid the extension width of a ceiling support structure and enlarge an effective filtering plane by providing a fluid groove on a filter element side, or inside the contour of the ceiling carrying structure, where a filter element is mounted.

A supporting wall 67 forms a fluid groove 70 together with a horizontal small strip 65 and a wall 62, inside which sealing medium is stored. Another filter supporter 4a has a corresponding fluid groove 71 to absolutely ensure seal of a filter element to a supporting structure, especially to its carrying rail 1, with both fluid grooves 70, 71. Both fluid grooves 70, 71 are structural components of a filter supporter 4 or 4a, and so the fluid groove 71 is located inside a supporting frame where a filter unit is mounted. In this way, the need to enlarge the fluid groove supporting frame formed by the supporting rail 1 and connecting members is eliminated, resulting in an effective filtering plane to be further enlarged, rather than to be reduced.