PURPOSE: To realize a stable control and to improve a product yield by providing a changing-rate restricting means, for restricting the time changing-rate of a speed correction value to a ≤prescribed value, to the back of a correction-value computing means.

CONSTITUTION: Because of the large deviation between a loop standard and a present loop-value at the time of starting a loop control, a loop controller 11 outputs a correction signal corresponding to it. After receiving the correction signal, a changing-rate limiter 13 outputs a speed-correction signal. Accordingly, a motor speed increases gradually in conformity with the speed correction signal, and an armature current is increased by piling up a portion, necessary for increasing speed, on a load current. Successively, a proportional plus integral acts in linear to set stably a loop control.