A large-diameter-side end surface (104a) of a cotter (104) can be supported by a cotter holder (32) from below through inserting a shaft member (102) along an inner periphery of the cotter (104) from above the cotter (104). In this state, a protrusion (104b) of the cotter (104) and an annular groove (102a) of the shaft member (102) are fitted to each other, and the cotter (104) and a retainer (103) are taper-fitted to each other. Consequently, the cotter is prevented from being unstable, and hence it is possible to assemble the retainer (103) and the cotter (104) to the shaft member (102) with good accuracy.