A steering system, comprising: a steering shaft; a steering column having the steering shaft passed therethrough to support the steering shaft; a first bracket fixed to the steering column; a second bracket, which includes a pair of fastening side plates that fasten a pair of fastened side plates provided in the first bracket or on the steering column, and which is attached to a vehicle body; and a lock mechanism, which includes a fastening shaft that is passed through elongate holes formed in the pair of fastening side plates so as to extend in a longitudinal direction serving as a position adjustment direction, and a pair of fastening members (33, 36) supported by the fastening shaft, and which fastens the pair of fastening members (33, 36) onto the pair of fastening side plates via the fastening shaft to lock a position of the steering column; wherein the lock mechanism includes a pair of first toothed portions provided on both sides of a pair of longitudinal edges of the elongate hole on an outer surface of at least one of the fastening side plates, and projecting in a lateral direction of the elongate hole, and a tooth member interposed between a corresponding one of the fastening members (33, 36) and a corresponding one of the fastening side plates and including a pair of second toothed portions meshing with the pair of first toothed portions, and the tooth member includes a body portion that is pressed in a fastening shaft direction by the corresponding fastening member, a pair of second toothed portion forming portions forming the pair of second toothed portions extending along the outer surface of the corresponding fastening side plate, characterized by a motion translation mechanism that translates movement of the body portion in the fastening shaft direction, which occurs when the body portion is pressed by the corresponding fastening member, into extension movement of the tooth member, in which the tooth member is extended so as to move the pair of second toothed portion forming portions away from each other obliquely with respect to the lateral direction, along the outer surface of the corresponding fastening side plate, wherein the motion translation mechanism is a pair of leg portions each connecting the body portion and a corresponding one of the second toothed portion forming portions, the tooth member is mountain-folded along first fold lines between each of the leg portion and the body portion, and is valley-folded along second fold lines between each of the leg portions and each of the second toothed portion forming portions, and the first fold lines and the second fold lines are parallel to the outer surface of a corresponding fastening side plate and extend in a direction tilted with respect to the longitudinal direction. a steering shaft; a steering column having the steering shaft passed therethrough to support the steering shaft; a first bracket fixed to the steering column; a second bracket, which includes a pair of fastening side plates that fasten a pair of fastened side plates provided in the first bracket or on the steering column, and which is attached to a vehicle body; and a lock mechanism, which includes a fastening shaft that is passed through elongate holes formed in the pair of fastening side plates so as to extend in a longitudinal direction serving as a position adjustment direction, and a pair of fastening members (33, 36) supported by the fastening shaft, and which fastens the pair of fastening members (33, 36) onto the pair of fastening side plates via the fastening shaft to lock a position of the steering column; wherein the lock mechanism includes a pair of first toothed portions provided on both sides of a pair of longitudinal edges of the elongate hole on an outer surface of at least one of the fastening side plates, and projecting in a lateral direction of the elongate hole, and a tooth member interposed between a corresponding one of the fastening members (33, 36) and a corresponding one of the fastening side plates and including a pair of second toothed portions meshing with the pair of first toothed portions, and the tooth member includes a body portion that is pressed in a fastening shaft direction by the corresponding fastening member, a pair of second toothed portion forming portions forming the pair of second toothed portions extending along the outer surface of the corresponding fastening side plate, characterized by a motion translation mechanism that translates movement of the body portion in the fastening shaft direction, which occurs when the body portion is pressed by the corresponding fastening member, into extension movement of the tooth member, in which the tooth member is extended so as to move the pair of second toothed portion forming portions away from each other obliquely with respect to the lateral direction, along the outer surface of the corresponding fastening side plate, wherein the motion translation mechanism is a pair of leg portions each connecting the body portion and a corresponding one of the second toothed portion forming portions, the tooth member is mountain-folded along first fold lines between each of the leg portion and the body portion, and is valley-folded along second fold lines between each of the leg portions and each of the second toothed portion forming portions, and the first fold lines and the second fold lines are parallel to the outer surface of a corresponding fastening side plate and extend in a direction tilted with respect to the longitudinal direction.