Magnetic poles are arranged such that the N pole and the S pole alternate in a circumferential direction. Two N poles face each other with an interval of 180° therebetween, and two S poles face each other with an interval of 180° therebetween. Each of the magnetization center of a North magnetic pole and the magnetization center of a South magnetic pole is positionally shifted from the center position between the magnetization centers of two magnetic poles of the other polarity located on opposite sides of the North or South magnetic pole. The magnetization centers of the magnetic poles are disposed in such a manner as to be positionally shifted from geometric centerlines. Brushes are disposed along the respective geometric centerlines. The magnetization centers of two adjacent magnetic poles are positionally shifted toward or away from each other by the same shift angle α with respect to the respective geometric centerlines.