PROBLEM TO BE SOLVED: To eliminate inserting position failure of a shaft inserted into a yoke. SOLUTION: A cover member 13 is mounted so as to cover a shaft connecting part 10a with which the yoke 10 is furnished, and an L shaped projected piece 13c projected in the inner direction of the shaft connecting part 10a is provided on the cover member 13. A pinion shaft 12 forms a groove part 12f on a width across flat part 12a. The groove part 12f has width extending to both sides in the shaft lengthy direction around a position against which the width across flat part 12a faces as its center in the case when the width across flat part 12a is arranged at a specified position in the inside of the shaft connecting part 10a. The inserting position failure is eliminated as the width across flat part 12a of the pinion shaft 12 cannot be inserted except for the time when the projected piece 13c is not inserted into the groove part 12f in the case when the width across flat part 12a of the pinion shaft 12 is inserted from an opening part 10g of the shaft connecting part 10a. COPYRIGHT: (C)2004,JPO