A tightened shaft portion 51a of a shaft 51 and a tightened portion 42b of a lower joint yoke 42 are coupled with each other such that a required torque can be transmitted therebetween by inserting the tightened shaft portion 51a of the shaft 51 into a shaft insertion hole 42b2 of the lower joint yoke (a coupling member) 42, and inserting a bolt 44 into a bolt insertion aperture 42b3 of the lower joint yoke 42 and a bolt engagement groove 51a1 of the shaft 51 and tightening the bolt in the condition (the regular position condition) that the bolt engagement groove 51a1 of the shaft 51 and the bolt insertion aperture 42b3 of the lower joint yoke 42 are axially aligned with each other. An insertion restriction portion 51b having a diameter smaller than that of the tightened shaft portion 51a is provided at the tip end of the tightened shaft portion 51a of the shaft 51 for restricting the insertion of the bolt 44 through the bolt insertion aperture 42b3 of the lower joint yoke 42 in the condition (the non-regular position condition) that the insertion restriction portion 51b axially overlaps the bolt insertion aperture 42b3 of the lower joint yoke 42 when the tightened shaft portion 51a is inserted into the shaft insertion hole 42b2 of the lower joint yoke 42, and the insertion restriction portion 51b can be rotated relative to the shaft 51 by a set torque smaller than the required torque.