An alternator includes a rotor and a stator having a stator core surrounding the rotor, and a polyphase stator winding installed in the stator core, the stator core being provided with an abutting portion extending axially making the stator core into an annular shape by abutting end portions of the stator core, the polyphase stator winding comprising a number of winding portions in which long strands of wire are wound so as to alternately occupy an inner layer and an outer layer in a slot depth direction within slots at intervals of a predetermined number of slots, the strands of wire folding back outside the slots at axial end surfaces of the stator core, and an insulating member being interposed between the stator core and straight portions of the strands of wire in the winding.