A plurality of teeth of an armature core arranged in a circumferential direction each include an inner winding portion in its proximal region and first and second branch portions that extend radially and branch off in a bifurcated manner from a distal end of the inner winding portion in the circumferential direction. After winding the first to ninth coils around the inner winding portions of all of the teeth, the tenth to eighteenth coils are wound sequentially around outer winding portions of adjacent ones of the teeth each including the first branch portion of one of the two adjacent teeth and the second branch portion of the other one of the adjacent teeth.