A filter device (10), especially an air filter for an internal combustion engine, includes a main filter element (22) arranged in a filter housing (12). The main filter element is flown through by a fluid to be filtered. The main filter element (22) is a multi-bellows filter having radially nested filter bellows (32, 34). Between the filter bellows (32, 34) a flow space (44) for the fluid is formed communicating with an end face of the main filter element (22), wherein each one of the filter bellows (32, 34) is embodied as a star-shaped filter with filter folds arranged in a star shape. On at least one end face of the main filter element (22) there is a seal (36) between one of the filter bellows (32, 34) and the filter housing (12). The main filter element (22) is forced seal-tightly against the filter housing (12) by a lateral insert (38) insertable into a housing receptacle (66) of the filter housing (12), or the main filter element (22) is seal-tightly forced by means of a locking device (74) against the filter housing (12), or the main filter element (22) is sealed by means of a two-component synthetic seal (36, 96, 98) relative to the filter housing (12,104).