a stator having a plurality of main magnetic poles each of which has a plurality of small stator teeth formed on a tip of a jutting section thereof, a core-back portion that connects outer portions of said main magnetic poles as a single piece, and a plurality of windings wound around the respective main magnetic poles; and

two sets of rotor units that are arranged in an axial direction and face said stator with an air gap therebetween,

wherein each rotor unit consists of two rotor cores that are separated in the axial direction and a magnet that is sandwiched by said two rotor cores and magnetized in the axial direction, each of said rotor cores has a plurality of small rotor teeth around its outer surface,

wherein said two rotor cores of each rotor unit are deviated by ½ pitch of said small rotor teeth in the circumferential direction, and said two rotor units are arranged to make the magnetic polarities of said small rotor teeth of the adjacent two rotor cores identical, and

wherein the relationship between a magnet thickness Tm and a rotor core thickness Tc satisfies 0.25≤Tm/Tc≤0.45,

wherein said magnets are ferrite magnet and the maximum magnetic flux density of said small rotor teeth in a radial direction component falls within a range of 1.4 T to 1.7 T, and

wherein said magnets are ferrite magnet and the maximum differential magnetic flux density, which is the maximum difference between the magnetic flux density at both ends and that at a center of said stator main magnetic pole in the axial direction, falls within a range of 0.75 T to 0.85 T.