To prevent generation of car body vibration and resonance sound by easily centering a spacer to a drive shaft without specially machining the spacer and the drive shaft.

A retractable spring 14 is provided with a cylindrical part 14a fittable to the inner surface of a first collapsible spacer 12 and allowing free insertion of shaft part 3a of a drive pinion 3, and elastic plate parts 14b, 14b and 14b formed by obliquely projecting a part of the cylindrical part 14a like a plate toward the center of the cylinder from three portions spaced from each other substantially equally on the same circumference of the cylindrical part 14a. A hemispherical projecting part 14c is formed on the center side surface of the cylinder of the forward end region of each plate part 14b. The retractable spring 14 is interposed between the shaft part 3a of the drive pinion 3 and the first collapsible spacer 12, and the cylindrical part 14a, that is, the first collapsible spacer 12 is centered to the shaft part 3a of the drive pinion 3.