A filter unit comprising an inlet (12) and an outlet (13) for a flow medium to be filtered, a first filter cartridge (14) having a first filter element, a second filter cartridge (15) having a second filter element, and a switching device (16) which allows alternating filter operation of the first or the second filter cartridge (14, 15). It is proposed that the first filter cartridge be a main cartridge (14) with a fine first filter element and the second filter cartridge be an auxiliary cartridge (15) with a coarse second filter element. The pressure loss in the auxiliary cartridge (15) is set to the same threshold value as a contaminated main cartridge (14), at which the differential pressure switch triggers an alarm such that during changing of the main filter, the alarm continuously sounds and is only switched off after changing the filtering back to a fine filtering operation.