A friction stir welding method and a structural body formed using a friction stir welding method. In particular, a friction stir welding method and a structural body made of an aluminium alloy extruded frame member formed by friction stir welding of a number of extruded frame members. Joining is performed by abutting a first member with a second member by contacting a downwards facing surface of an overlapping portion of a first member which projects above one face of the other member in the lateral direction in and an upwards facing surface of a second member in such manner that the downwards facing surface and the upwards facing surface are in slideable contact before joining and the first and second member are rotatable, and carrying out a friction stir welding to join the members to form a frame structure.