A vehicle alternator for generating electric power is driven by an engine mounted on a vehicle driven through a driving belt. The alternator has a pulley part joined to and directly receiving a driving force from the engine through the driving belt, a field magnet part located in separation from the pulley part, a rotation speed changing part placed between the pulley part and the field magnet part. The rotation speed changing part converts the number of revolutions between the pulley part and the field magnet part. The rotation speed changing part decreases a speed changing ratio “S” according to increasing of the number of revolutions of the engine, where the speed changing ratio “S” is obtained by dividing the number of revolutions of the field magnet part by the number of revolutions of the pulley part.