A rotor is operable to rotate in a rotating direction, and is provided with five teeth that define five slots therebetween at regular intervals in the rotating direction. A pair of arc-shaped magnets surrounds the rotor, and faces each other through the rotor. A value of a circumferential angle between two adjacent slots with respect to an axis of the rotor is defined as a. A value of a circumferential angle between opposite end edges of an inner surface of the magnet in a circumferential direction of the magnet, which faces the rotor, with respect to the axis of the rotor is defined as b. The value of b falls within a range from 23 to 25 when the value of a is assumed as 12.