To provide a sensor for detecting abnorxtaality in motor accurately, by detecting an abnormal state of a current measuring device itself when it is out of order.

A current in a motor 10 and a voltage applied to the motor 10 are measured, and the operation state of the motor 10 is detected. The current carried at the motor 10 is estimated on the basis of the measured voltage and the operation state detected. Then, the abnormality of the motor 10 is detected on the basis of the measured current and the estimated current. If a current maeasuring device is out of order, the abnormality is detected and thereby an abnormal state of the motor 10 is detected accurately. At the sajne time, the abnormality in the current measuring device 31 or the abnormaljty caused by a missing connection between the motor 10 and a power strpply is also detcted.