An electromagnetic clutch has a drive member secured to a crankshaft of an internal combustion engine, and a driven member adjacent to the drive member. The system comprises a circuit for detecting the difference between numbers of revolutions of the drive member and driven member, a comparator for producing an output when the difference is higher than a reference value, a timer operated by the output of the comparator, and a warning device such as a buzzer operated by the signal from the timer when a predetermined time elapses. Thus, when the clutch slips, the warning signal is generated. Further, a circuit for changing clutch engaging characteristics is provided to be operated by the output of the comparator for preventing the overheating of the clutch.