A pair of parallel pins abutting against the inner peripheral surface of a hollow shaft are provided to a first arm part of a shank at a predetermined spacing in the longitudinal direction. This prevents the shank from tilting or rattling. Further, an insertion hole of the retaining cap and the cross section of the first arm part each have an elliptical shape, and thus the rotation of the shank with respect to the hollow shaft is restricted and the position of the shank is kept such that the interlocking part points upward.