Each of the commutator pieces 9 and each of the coils 7 are connected by a power supply circuit 42. FIG. 10 is a circuit diagram illustrating a structure of the power supply circuit 42. The four coils 7 are annularly connected. In addition, the twelve commutator pieces 9 are short-circuit connected to the annular coil circuit every 360/p degrees (that is, every 120 degrees) with respect to the rotation angle of the rotor 6. For example, on the annular coil circuit illustrated in FIG. 10, a point Q₁ between the first coil and the second coil is short-circuit connected to commutator pieces C₁, C₅, and C₉.

Positions of these commutator pieces C₁, C₅, and C₉ are shifted by 120 degrees with respect to a rotation axis C. Therefore, a potential of the point Q₁ becomes identical every time the rotation axis C rotates by ?. Similarly, a point Q₂ between the second coil and the third coil is short-circuit connected to commutators pieces C₂, C₆, and C₁₀. In addition, a point Q₃ between the third coil and the fourth coil is short-circuit connected to commutator pieces C₃, C₇, and C₁₁, and a point Q₄ between the fourth coil and the first coil is short-circuit connected to commutator pieces C₄, C₅, and C₁₂. In addition, positions of brushes B₁ and B₂ are shifted by 180 degrees with respect to the rotation axis C.