PURPOSE: To reduce cogging torque and to suppress vibration, noise and fluctuation in the rotation.

CONSTITUTION: A rotor core 26 has outer peripheral surface 26 formed such that the dimension of an air gap 32 between the outer peripheral surface 26a and the inner peripheral surface 22a of a stator core 22 increases gradually from a part corresponding to the peripheral center of pole toward a part corresponding to the center between poles thus setting the dimension Gb of air gap between poles two times as long as the dimension Gb of air gap in the center of the pole. Since the air gap 32 increases toward the center between the poles and the magnetic resistance increases proportionally, the flus density in the air gap becomes sparse at the part corresponding to the center between the poles and concentrates at the part corresponding to the central part of the pole thus bringing about a substantially sinusoidal distribution. This structure reduces cogging torque.