A steering system comprising: a steering operation shaft provided so as to be able to reciprocate in an axial direction and having a groove (56, 57) that is formed in part of an outer periphery of the steering operation shaft and along which a ball rolls; a motor disposed parallel to the steering operation shaft; a drive pulley coupled to the motor; a driven pulley rotatably provided in a housing that accommodates the steering operation shaft; a belt wound around the drive pulley and the driven pulley; a ball screw nut loosely fitted into an inner periphery of the driven pulley and constituting a ball screw mechanism that is screwed onto the steering operation shaft via a plurality of balls and converts rotation of the driven pulley into reciprocation of the steering operation shaft; and an aligning structure including a plurality of mounting grooves formed on a first surface among an inner peripheral surface of the driven pulley and an outer peripheral surface of the ball screw nut and extending in an axial direction, a plurality of locking grooves formed on a second surface among the inner peripheral surface of the driven pulley and the outer peripheral surface of the ball screw nut, extending in the axial direction, and opposed to the respective mounting grooves in a radial direction, and a plurality of elastic members each of which has a pair of base parts (63a, 63b) extending in the axial direction and a spring part formed between the base parts (63a, 63b) and being elastically deformable in the radial direction, the aligning structure being configured to apply biasing force from the elastic members to between the driven pulley and the ball screw nut so as to align an axis of the driven pulley and an axis of the ball screw nut with each other, wherein each of the elastic members is disposed in the corresponding mounting groove with at least one of the pair of base parts (63a, 63b) spaced apart from a circumferential side surface of the mounting groove, and the spring part is locked with respect to the corresponding locking groove from both sides in a circumferential direction, characterized in that in the driven pulley and the ball screw nut, positioning portions (71, 72) are each formed that determine a relative position between the corresponding mounting groove and the corresponding locking groove such that the mounting groove and the locking groove are opposed to each other in the radial direction. a steering operation shaft provided so as to be able to reciprocate in an axial direction and having a groove (56, 57) that is formed in part of an outer periphery of the steering operation shaft and along which a ball rolls; a motor disposed parallel to the steering operation shaft; a drive pulley coupled to the motor; a driven pulley rotatably provided in a housing that accommodates the steering operation shaft; a belt wound around the drive pulley and the driven pulley; a ball screw nut loosely fitted into an inner periphery of the driven pulley and constituting a ball screw mechanism that is screwed onto the steering operation shaft via a plurality of balls and converts rotation of the driven pulley into reciprocation of the steering operation shaft; and an aligning structure including a plurality of mounting grooves formed on a first surface among an inner peripheral surface of the driven pulley and an outer peripheral surface of the ball screw nut and extending in an axial direction, a plurality of locking grooves formed on a second surface among the inner peripheral surface of the driven pulley and the outer peripheral surface of the ball screw nut, extending in the axial direction, and opposed to the respective mounting grooves in a radial direction, and a plurality of elastic members each of which has a pair of base parts (63a, 63b) extending in the axial direction and a spring part formed between the base parts (63a, 63b) and being elastically deformable in the radial direction, the aligning structure being configured to apply biasing force from the elastic members to between the driven pulley and the ball screw nut so as to align an axis of the driven pulley and an axis of the ball screw nut with each other, wherein each of the elastic members is disposed in the corresponding mounting groove with at least one of the pair of base parts (63a, 63b) spaced apart from a circumferential side surface of the mounting groove, and the spring part is locked with respect to the corresponding locking groove from both sides in a circumferential direction, characterized in that in the driven pulley and the ball screw nut, positioning portions (71, 72) are each formed that determine a relative position between the corresponding mounting groove and the corresponding locking groove such that the mounting groove and the locking groove are opposed to each other in the radial direction.