A speed region of a motor is divided into a region lower than a first switch speed VS1, a region higher than a second switch speed VS2 and a region between the first and second switch speeds. An abnormal load determination level to be compared with an estimated load obtained by an observer is set for each of the regions. An abnormal load determination level AL4 applied when an acceleration exceeds a threshold value is set. The abnormal load determination level AL4 is used for comparison with the estimated load until a predetermined period of time elapses after the acceleration to be commanded to the motor, which once exceeded the threshold level, drops. In other cases, the estimated load is compared with the abnormal load determination level having a value according to the speed. If the estimated load exceeds the compared abnormal load determination level, it is judged that a collision is detected.