Each magnet 5 is a permanent magnet, and has a cuboid shape. The magnet 5 has a rectangular lateral face having long sides and short sides. As shown in FIG. 1, these magnets 5 are disposed in accordance with a predetermined rule along the rotating direction of the rotor 3 (the direction of the arrow in FIG. 1). Specifically as shown in FIG. 2, magnet groups 10 each consisting of a left magnet 5L, a middle magnet 5M, and a right magnet 5R are disposed at the 45° intervals along the rotating direction of the rotor 3. Each magnet 5 is the left magnet 5L, the middle magnet 5M, or the right magnet 5R. The left magnet 5L and the right magnet 5R on both sides of the middle magnet 5M refer to the illustrated positions of these magnets 5 for the purpose of illustration. These magnets may include magnets 5 arranged on one side and on the other side of the middle magnet 5M.

In this magnet group 10, the middle magnet 5M has the N-pole on the side adjacent to the stator 2 and the S-pole on the other side. The left magnet 5L and the right magnet 5R are disposed so that they have mutually reversed polarity of the adjacent magnetic poles of the middle magnet 5M. Specifically the left magnet 5L is closer to the S-pole of the middle magnet 5M than the N-pole, and so has the N-pole on the side adjacent to the middle magnet 5M. Similarly, the right magnet 5R is closer to the S-pole of the middle magnet 5M than the N-pole, and so has the N-pole on the side adjacent to the middle magnet 5M.