Patent Document 1 discloses a technology in which, by carrying out a friction stir welding from the front and back surfaces of the butting portion of the metal members in such a way that the plasticized regions formed in the friction stir welding are in contact with each other, the butting portion is joined without a gap. According to this technology, air-tightness and water-tightness of the metal members that are joined together can be enhanced.
On the other hand, Patent Document 2 discloses a technology in which a friction stir welding is carried out by inserting a rotary tool specialized for inner corner friction stir welding into an inner corner of two metal members that are butted against each other orthogonally. FIG. 37 is a cross-sectional view showing a conventional friction stir welding method. In the conventional friction stir welding method, a friction stir welding is carried out by the rotary tool 110 specialized for inner corner friction stir welding at the butting portion J formed by the end surface of the metal member 101 and the side surface of the metal member 102 butted against each other. The rotary tool 110 specialized for inner corner friction stir welding includes a holding block 111 in the shape of a triangle prism, a stirring pin 112 which is rotatable relative to this holding block 111 while penetrating through the holding block 111. In joining, the stirring pin 112 is rotated, while the holding block 111 is in contact with respective side surfaces of the metal members 101, 102.