The present disclosure relates to a device and to a method for detecting filter soiling of a filter system having at least an air filter installed in an air duct and an electrically controlled fan for producing an air flow in the air duct, wherein the device comprises at least a motor controller for controlling the fan motor by means of PWM control and an evaluating apparatus for determining the actual rotational speed (ni) of the fan in accordance with the actual degree of modulation (Ti) of the motor current for achieving the present rotational speed (ni), wherein, in the evaluating apparatus, setpoint value pairs (nsetpoint, Tsetpoint) for the setpoint rotational speed (nsetpoint) at a certain degree of modulation (Tsetpoint) are also stored, as well as a comparator in order to compare the actual value pairs (ni, Ti) with the setpoint value pairs (nsetpoint, Tsetpoint) and to obtain the degree of filter soiling of the air filter therefrom.