<p>A rotation adjustment mechanism is mainly constituted of a coil spring serving as an elastic member provided in a transmission path of motive energy extending from an alternator pulley to an alternator shaft and a clutch mechanism including a function of changing over transmission and shut of the motive energy. The motive energy of the alternator pulley is transmitted from the coil spring through the clutch mechanism in a motive energy transmission mode to the alternator shaft. The coil spring reduces a minute rotary variation occurring on at least one of the alternator pulley and the alternator shaft by elastic deformation. The clutch mechanism is provided in the middle of the transmission path from the coil spring to the alternator shaft and shuts the transmission of the motive energy from the alternator pulley to the alternator shaft when the number of rotations of the pulley is lower than that of the shaft by a predetermined value or more.</p>
