Note that both ends of the magnetized region when a plurality of magnetic poles is magnetized in the ring magnet or the single magnet piece have an angle at which the magnetic flux density decreases from a center position (magnetization center position) of each magnetic pole, and the magnetic flux density first becomes zero. The relationship between a magnetic flux density distribution of the ring magnet and the magnet angle θ_M is illustrated in FIG. 14. Angles θ₀ to θ₁₂ in FIG. 14 represent angles at which the magnetic flux density becomes zero. As illustrated in FIG. 14, a non-magnetized region or a minute polarity inversion region may be generated in the vicinity of 0° (θ₀ to θ₁), 90° (θ₂ to θ₄), 180° (θ₅ to θ₇), and 270° (θ₈ to θ₁₀), which correspond to a switching point with an adjacent magnetic pole, and the magnet angle θ_M is defined excluding such ranges. In FIG. 14, as indicated by θ₁ to θ₂, θ₄ to θ₅, θ₇ to θ₈, and θ₁₀ to θ₁₁, an angular range from when an absolute value of the magnetic flux density exceeds zero to when the magnetic flux density includes the center position of each magnetic pole and reaches 0 again is defined as the magnet angle θ_M. In any case, the angle formed by the magnetized region of one magnet magnetic pole 5 and the rotation axis C in the cross section perpendicular to the rotation axis C is defined as the magnet angle θM.