A Belt Driven Electric Starter System (BES) may have an idle pulley is composed of pivot point idle pulley forming pivot points at both sides of driving belt, which is wound on starter generator pulley and crank pulley, and driven idle pulley forming the fixing point, and a side of tension controller is fixed to engine and the opposite side is fixed to starter generator combined with starter generator pulley, such that it is possible to more effectively use the empty space without interference with the other portions in the entire layout of the BES. Further, excessive tension of driving belt due to starter generator when the engine is started can be attenuated by retraction of tension controller that moves with starter generator, while the excessive tension of driving belt due to crank pulley when electricity is generated can also be attenuated by extension of tension controller that moves with starter generator.