PURPOSE: To reduce the noise, which occurs at rotation of a pole core, by forming a plurality of fan-shaped cylinders, which have through holes in axial direction, at specified intervals around the end faces of bobbins, and arranging each cylinder in the recess being formed in each root part of the claws contiguous in circumferential direction.

CONSTITUTION: A randel type pole core 4, where a field coil 3 is wound through a resinous bobbin 2, is fixed to the periphery of a shaft 1. The pole core 4 comprises the core 5 on the front side and the core 6 on the rear side, which have six claws 5a and 6a each at the peripheral margin, and cylinders 5b and 6b as yokes are formed at the center of each core 5 and 6. And the cylinders 5b and 6b of each core 5 and 6 are butted to each other, and those are assembled so that the mutual claws 5a and 6a may engage with each other. Hereby, the irregularity on the periphery of the pole core 4 can be made small, and at rotation of the pole core 4, the air discharged in the centrifugal direction from each recess 5c and 6c decreases, so whizzing accompanying pressure change or the collision sound against the stator coil end can be reduced.