Abutted portions of face plates 12b and 22b of frame members 10 and 20 are friction stir welded. With the abutting, a projecting portion 20c of an end portion of the face plate 20b of the frame member 20 is inserted into a recessed portion 12c of an end portion of the face plate 12b of the frame member 10. By this engagement, the outer face on one face side of the face plates 12b and 20b form a smooth surface, so that there is no step at the joined edges. Next, a rotary tool 250 is inserted into the gap between the abutted plates, from the side of the raised portions 16 and 26 of the face plates 12b and 20b, while the other faces 12bc and 20bc of the face plates 12b and 20b are supported flatly on a bed during the friction stir welding. Accordingly, the welding of the engaged plates can be carried out without any step-wise difference.