Abstract: A shaft having a diameter A is press-fitted into the central hole of the boss. A tapered portion is formed around one end of the shaft, which is inserted to the center hole, such that a dimension in the extending direction of the shaft is R. The boss includes a cylindrical recess formed concentrically with the center hole on one end face thereof facing a side from which the shaft is inserted such that a dimension in the extending direction of the center hole thereof is larger than R, and such that a diameter thereof is A<B≦1.05A. The cylindrical recess includes a first side wall linearly extending from the end face of the boss and a second side wall subsequent to the first side wall, which is tapered inwardly in order to serve as an insertion guide member against which the tapered portion of the shaft is to be abutted when the shaft is inserted into the center hole.