PURPOSE: To enable highly responsive detection of abnormality by providing a magnetic clutch interposed between a motive power side pulley and a driven side shaft, a magnetic clutch driving control circuit and a torque detecting means.

CONSTITUTION: A control part 30 receives the torque value from a torque sensor 31 and computes a variation between the torque value (zero) at the time of a magnetic clutch driving signal not being inputted from an amplifier 20 and the torque value at the time of the magnetic clutch driving signal being inputted. The control part 30 also judges whether or not the fluctuating part of the torque value is over the specified threshold value. In case the fluctuating part of the torque value is not over the threshold value, the magnetic clutch 10 is judged to be in failure, and failure detection signals as control signals are outputted to a vacuum switching valve(VSV) 32 and a failure detecting lamp 33 so as to stop idling-up and light a lamp 33.