A pair of pole cores comprises shoulder portions positioned on both axial ends of the diametrically outermost portions of the claw-shaped magnetic poles formed into beveled or curved outer circumferential surfaces, and diametrically outermost leading-edge face portions positioned on the leading edges in the direction of rotation at both axial ends of the diametrically outermost portions of the claw-shaped magnetic poles each being formed into an outer circumferential surface inclined in a circumferential direction by a surface which passes through a point A where the inner circumferential edge of the diametrically outermost leading-edge face positioned on the leading edge in the direction of rotation of the diametrically outermost portion of each of the claw-shaped magnetic poles intersects a plane which passes through an axial end of the stator core and perpendicularly intersects the axis of the shaft, and a point B located on a ridge line where the diametrically outermost leading-edge face intersects the beveled or curved outer circumferential surface formed on the shoulder portion.