A motor-driven power steering system includes a worm shaft unit having a worm gear, a worm wheel unit rotatably engaging with the worm gear, a housing unit having a worm receiving part formed therein to receive the worm shaft unit, and a shaft support unit having a support part located between the housing unit and the worm shaft unit to support the worm shaft unit, and a shaft pressing part located between the support part and the worm shaft unit to press the worm shaft unit towards the worm wheel unit.