To detect the occurrence of a slip in a slip clutch part without using a sensor for measuring a rotating speed.

When the slip is caused in the slip clutch part 3 interposed between an input shaft 1 and an output shaft 2, rotation of the input shaft 1 and rotation of the output shaft 2 become asynchronous, and a phase difference is generated between both. An operation converting part 4 is constituted for converting this phase difference into linear displacement of an actuator 43, and the effect of causing the slip is known by detecting displacement of the actuator 43 by a detecting part 5.