To join a driving shaft and the inner surface of a socket mounted on the driving shaft at not only a part in one specified position in the high direction of the driving shaft but also a part having the specified height on the root part of the driving shaft.

In a case where a socket 3 is mounted on a driving shaft 2, the driving shaft 2 and the inner surface 3a of the socket 3 are joined to each other at two parts of B-B and C-C. Accordingly, the socket 3 does not yield to by fastening force even if a bolt and a nut are tightly fastened or the bolt and the nut fastened by strong force are loosened. As a result, the bolt and the nut can be tightly fastened and also the bolt and the nut fastened by strong force can be loosened.