At least one circumferential slot is provided at a predetermined axial position on an armature core so as to cross axial slots receiving a plurality of coil sides of an armature winding and to communicate adjacent axial slots, and the plurality of coil sides to be generally mounted in the same axial slot are partially shifted from one slot to an adjacent one through the circumferential slot so that the coil sides of the armature winding are partially arranged to extend through adjacent slots at a fixed, predetermined circumferential distance. In this manner, the generation of commutating spark is substantially prevented and the commutating performance is improved.