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