To prevent stress concentration through an improvement of a joint structure of friction stirring joining.

Edge faces of two members 100, 120 to be joined together by a friction stirring joining method are butted to form a joining face 140 of a joint part. A first member 100 has a protruded part 110 protruded from an upper surface of the member 100 on the side to which a friction stirring joining tool 50 is inserted. A side face 112 of the protrusion 110 is connected through a small arc R2 and a large arc R3 to the upper surface 102 of the member 100. A second member 120 also has a similar structure. The friction stirring joining tool 50 having a small diameter part 52 in the top, performs the friction stirring joining, while fitting the center of the small diameter part 52 to the joining face 140 (b). In the joint part of the joined members 100, 120, a flat surface 150 is formed on the upper surface with burrs B1 remaining in the both edge parts. The joint part is completed by removing the burrs B1 by cutting(c). The connecting part has the arc R3 with a large curvature so that stress concentration is not caused.