The invention provides a steering system that is capable of suppressing backlash of a pulley and a reduction in strength due to temperature variations even if the pulley which is made of a resin is externally fitted with a nut member made of metal. The steering system includes: a rack shaft that is movable to be advanced and retracted in the axial direction; a cylindrical nut member made of metaland threadably engaged with a spiral screw groove formed in the outer peripheral surface of the rack shaft via a plurality of balls; a pulley made of a resin and externally fitted with the nut memberso as not to be rotatable relative to the nut member; a belt wound around the pulley; an electric motor that rotates the pulley and the nut member via the belt; and a retention ring, a plate spring portion of which is disposed between the outer peripheral surface of the nut member and the inner peripheral surface of the pulley in an elastically deformed state. The retention ring presses a second planar portion formed on the inner peripheral surface of the pulley radially outward using a restoring force of the plate spring portion.