The rotating body 3 includes a relative surface 3a, which is the surface of the cylindrical shape facing the facing surface 2a, and a circumferential surface 3b, which corresponds to the lateral surface of the cylinder. The circumferential surface 3b is a surface of revolution formed by rotating a straight line on a plane including a central axis Q (see FIGS. 6 and 12) with respect to the central axis Q, and is formed so as to have a diameter becoming larger as the diameter is farther away from the relative surface 3a. The inclination angle of the circumferential surface 3b with respect to the central axis Q is the same as the inclination angle of the outer circumferential surface 2b with respect to the central axis P. The rotating body 3 includes a bottom surface 3c that is circular and is formed on a surface of the cylindrical shape opposite to the relative surface 3a.

The rotating body 3 is formed such that the outer diameter of the relative surface 3a is equal to the outer diameter of the facing surface 2a or is larger than the outer diameter of the facing surface 2a to some extent, and that the diameter becomes larger from the relative surface 3a toward the bottom surface 3c. Therefore, the rotating body 3 has a larger outer diameter than the base body 2.