PURPOSE: To enable very precisely detecting of even slight belt slip occurred only at engine abruptly accelerating time and to enable remarkably easily comparing of detection signals thus detected.

CONSTITUTION: This device incroporates an alternator velocity detector 32 connected to both terminals of a battery for detecting a charging signal having a frequency corresponding to the velocity of the alternator. The output from the detector 32 is converted into an alternator velocity signal VA through a waveform shaping circuit 36 and an F-to-V converter 38. This signal VA is supplied to one input terminal of a comparator 39. The output of an engine velocity detector 40 connected to an ignition circuit is converted through a waveform shaping circuit 42, a frequency multiplier 44, and an F-to-V converter 46 into an engine velocity signal V, which is supplied to the other input terminal of the comparator 39. The comparator 39 produces a signal corresponding to the slip amount by the difference between the inputs from both the converters 38 and 46 to a peak holding circuit 48, which holds the maximum slip amount and indicate it in a display circuit 49.