Friction stir welding method and group of shape members for friction stir welding

The purpose of the present invention is to provide a method that can be easily applied to friction stir welding where the thickness of face plates at one welding portion is different from the thickness of face plates at another welding portion. Three shape members having two butt joint portions are mounted on a jig The face plate of shape member is thick and the face plate of shape member is thin. The protruded height of projection is greater at portion B where surface plates being welded are thin than the protruded height of projection A where surface plates are thick. The depths for inserting rotary tools to shape members for friction stir welding are the same. Friction stir welding is facilitated since the size and insertion depth of all the rotary tools being applied can be the same.