Disclosed is a motor-driven steering system capable of easily adjusting tension. The motor-driven steering system comprises a driven pulley connected to a driving pulley of the motor through a belt, a gear housing for surrounding the driven pulley, a motor housing rotatably connected to the gear housing by means of a hinge shaft, and a guide formed at one side portion of the gear housing. The distance between the driving pulley and the driven pulley is adjusted by rotating the motor housing relative to the gear housing, thereby providing the motor-driven steering system having a simple construction and capable of easily adjusting the tension of the belt.