According to the invention, an apparatus for detecting slip of a drive belt (4) of a generator (1) driven by a drive motor (2) is proposed, in which the slip of a drive belt (4) can be determined by observation of the on-board electrical system voltage (Ub) and of the signal course at the terminal (DF) of the generator (1). For determining the slip, the largest possible electrical load (10) is switched, and the voltage course of the on-board electrical system voltage (Ub) before and after the switching of the electrical load (10) is evaluated. By additional observation of the signal at the terminal (DF) of the generator (1), it is precluded that electrical consumers that are not observed could adulterate the result.