Construction is achieved that is able to effectively prevent the displacement in the axial direction of a ball bearing 16 that supports the middle section of a pinion shaft 6 so as to be able to rotate freely, and that is compact and low cost. A fastening groove 28b is formed around the entire circumference of a portion near an opening of the inner circumferential surface of a sub housing section 12c of a casing 10c, and a radially outside portion of a C-shaped retaining ring 29b is fastened in this fastening groove 28b. As a result, a side surface in the axial direction of a radially inside portion of the retaining ring 29b is pressed against a side surface in the axial direction of an outer ring 18 of a ball bearing 16 that is fitted inside and supported by the inner circumferential surface of the sub housing section 12c. Moreover, a clip 35 having a radially outside arm section 43 and radially inside arm section 44 is mounted in the opening edge section of the sub housing section 12c, and a diameter reduction prevention section 36 that comprises the tip-end section of the radially inside arm section 44 is inserted into a non-continuous area 34 of the retaining ring 29b.