In order to reduce undesired sound caused by circular ring oscillation in the diametric direction of the stator by an electromagnetic force, electromagnetic force drift memory stores a drift information of the electromagnetic force causing the circular ring oscillation of the stator. This electromagnetic force drift information is read out according to the position information &thgr; and is supplied to a correction coefficient generating circuitry. Correction coefficient generating circuitry corrects the amplitude component of the activation current wave from by forming correction coefficient &bgr; (t) of the activation current so as to cancel the electromagnetic force harmonic component in the diametric direction which component is the component of the circular ring oscillation of the stator.