A guide member 48 includes a cylindrical portion 52 fitted to a front end of a pinion shaft 19 and extending on the side of an opposite yoke and a slit engagement plate 54 extending on the side of the pinion shaft 19 from a portion, on the side of the pinion shaft 19, of the cylindrical portion 52 and engaging with a slit 42 of a yoke 22, the cylindrical portion 52 being inserted first in a clamp portion 36 of the yoke 22 upon the yoke 22 being fitted to the pinion shaft 19, the yoke 22 being rotatably guided by the cylindrical portion 52 to thereby improve workability of assembling the yoke 22.