Plural auxiliary devices such as a water pump and a generator coupled to a serpentine belt-drive system are driven by an internal combustion engine mounted on an automobile vehicle. The generator is divided into a first generator having a pulley in which a one-way clutch is installed and a second generator having a usual solid pulley. Fluctuations in the belt tension caused by changes in instantaneous rotational speed of the internal combustion engine are suppressed or reduced by the one-way clutch installed in the pulley of the first generator. An inertia torque of a rotor of the first generator may be made higher than that of the second generator because effects of the inertia torque on the belt tension fluctuations are alleviated by the one-way clutch.