A rack shaft is supported in an inner peripheral portion of a tubular housing of a gear housing via a rack bush. An inner peripheral portion of the rack bush is formed as a peripheral surface approximately similar to a circle in which a peripheral surface constituted by a large circular arc and a peripheral surface constituted by a small circular arc are continuously provided. The rack shaft is not in contact with the peripheral surface constituted by the large circular arc, and the peripheral surface constituted by the small circular arc is formed as a support portion of the rack shaft. The support of the rack shaft by the peripheral surface constituted by the small circular arc is not accomplished when a load applied to the rack shaft is nothing or low, but is accomplished when the load is high. Further, an engaging convex portion acting as a positioning and rotation prevention for the bush is provided in an outer peripheral portion of the rack bush.