A filter system for cleaning the air of an enclosure to be ventilated, especially for fiber-optic equipment, the enclosure including a ventilation opening and being associated with an insertion shaft located proximate to the ventilation opening, the filter system including (a) an insertion part including a filter housing for accommodating at least one filter element and further including a handle for inserting the insertion part into the insertion shaft, the filter housing being configured so as to cover the ventilation opening when the filter housing is in a working position within the insertion shaft; and (b) an insertion means provided at a first end portion of the insertion shaft for inserting the insertion part into the insertion shaft; characterized in (c) that the insertion means includes a neck portion defining the insertion opening for inserting the insertion part into the insertion shaft in a given insertion direction, the axial length of the neck portion corresponding to the axial length of the handle, and (d) that the neck portion with the insertion opening is stationary with respect to the enclosure, (e) the filter housing of the insertion part being insertable into the insertion shaft through neck portion in a predefined first angular insertion position only, in which the cross-section of the filter housing in planes perpendicular to the insertion direction is aligned with the cross-section of the insertion opening in planes perpendicular to the insertion direction of the insertion means, (f) the filter housing further being rotable about an axis parallel to the insertion direction from the first angular insertion position into a second angular position corresponding to the angular orientation of the working position after the filter housing of the insertion part has been fully inserted through the insertion opening, (g) the insertion opening having a rectangular cross-section in planes perpendicular to the direction of insertion, (h) the angular distance between the first and second angular position being 90 degrees. (a) an insertion part including a filter housing for accommodating at least one filter element and further including a handle for inserting the insertion part into the insertion shaft, the filter housing being configured so as to cover the ventilation opening when the filter housing is in a working position within the insertion shaft; and (b) an insertion means provided at a first end portion of the insertion shaft for inserting the insertion part into the insertion shaft; (c) that the insertion means includes a neck portion defining the insertion opening for inserting the insertion part into the insertion shaft in a given insertion direction, the axial length of the neck portion corresponding to the axial length of the handle, and (d) that the neck portion with the insertion opening is stationary with respect to the enclosure, (e) the filter housing of the insertion part being insertable into the insertion shaft through neck portion in a predefined first angular insertion position only, in which the cross-section of the filter housing in planes perpendicular to the insertion direction is aligned with the cross-section of the insertion opening in planes perpendicular to the insertion direction of the insertion means, (f) the filter housing further being rotable about an axis parallel to the insertion direction from the first angular insertion position into a second angular position corresponding to the angular orientation of the working position after the filter housing of the insertion part has been fully inserted through the insertion opening, (g) the insertion opening having a rectangular cross-section in planes perpendicular to the direction of insertion, (h) the angular distance between the first and second angular position being 90 degrees.