To prevent appearance of abnormal driving by executing a driving error judgement by a motor driving error judging means during driving operation and thereby monitoring an error periodically.

A ROM 510 as a storage means for storing data is connected to a CPU 509. In the ROM 510, necessary data such as a target speed table 511, a speed deviation allowable value table 512, and a position deviation allowable value table 513 are stored. The outputs of a speed deviation detecting section 507 and a position deviation detecting section 508 are compared with a speed deviation allowable value and a position deviation allowable value obtained from the speed deviation allowable table 512 and the position deviation allowable value table 513 respectively. In the case that either of the outputs exceeds the allowable value, it is judged that there is a driving error. By this method, various measured values which allow the estimation of the motor driving condition can be obtained periodically. By monitoring the magnitude of a difference between each measured value and the corresponding standard value preliminarily set at all times, the appearance of abnormal driving can be prevented.