On vehicles with automatic transmission, the shape of the pulley on the alternator with built-in vacuum pump is improper. Because of this, when the alternator drive belt is too tight after vehicle maintenance, the belt may prematurely wear. If left unfixed, in the worst case, the belt gets damaged stopping the vacuum pump, and this causes impaired function of the vacuum brake booster resulting in reduced brake efficiency. Incidentally, the belt has sufficient durability as long as it is adjusted within the specified range.