An alternator for use in an automobile is driven by an internal combustion engine. The alternator includes a pulley that is coupled to a rotating shaft of the engine via a driving belt such as a poly-V belt. Plural pulley grooves to be engaged with the driving belt are formed on the pulley between a front flange and a rear flange. A projected surface of a front frame is located around the outer periphery of the rear flange, so that the driving belt is correctly positioned between the front and the rear flanges, guided by the projected surface. Two overlaps in the axial direction between the pulley and the front frame are formed to reduce an amount of water entering into the alternator through a gap between the pulley and the front frame.
