To enhance a welding accuracy, and to miniaturize a device by thinning the meeting layer and the heat-affected layer of a joining part while securing the welding strength of axial members.

In the structure of a friction welding part performing butt welding by rotating relatively a first shaft 11 and a second shaft 21 each other on the same axial line while applying a pressure welding force in the axial line direction, a recessed part 13 is formed, which has a tapered inner peripheral surface 14 on the joining end face 12 of the first shaft 11, and a projecting part 23 is formed, which has a tapered outer peripheral surface 24 corresponding to the tapered inner peripheral surface 14 on the joining end face 22 of the second shaft 21. By welding the tapered inner peripheral surface 14 and the tapered outer peripheral surface 24, both shafts 11, 21 are subjected to friction welding.