A flat filter element has a filter medium and a sealing section that seals the flat filter element relative to a filter housing. A support element section is arranged on a circumferential side of the filter medium outside of the sealing section and supports the flat filter element relative to an inner side of the filter housing, wherein the support element section projects past the filter element in at least one direction. The filter housing has a mounting opening through which the flat filter element is inserted into the interior where the flat filter element is arranged such that the inlet is separated from the outlet of the filter housing. A projection arranged on an inner side of the filter housing supports under mechanical tension the support element section of the flat filter element in a mounting position of the flat filter element in the interior.