A joining method of a pair of workpieces, each workpiece having a non-flat portion and an abutting surface continuous to the non-flat portion, wherein the method provides an extension portion on each of the workpieces, the extension portion having a surface continuous with the non-flat portion and the abutting surface and projecting outward from the workpiece, and holding the workpieces so as to form a joining line between the non-flat portions of the respective workpieces by bringing the abutting surfaces thereof into contact with each other. Also disclosed is a joining tool having a substantially cylindrical shoulder portion and a projection projecting outward from a tip surface of the shoulder portion.