Two extruded shape members to be subjected to friction stir welding have joining ends butted against each other to form a joining surface of the joint portion. A first extruded shape member has a protrusion projecting from an upper surface of a flat panel portion toward a direction from which a friction stir welding tool is to be inserted. A side surface of the protrusion is connected to the upper surface of the flat panel portion via a small arc R2 and a large arc R3. A second extruded shape member is formed substantially in the same manner. The friction stir welding tool has a small diameter portion formed to the end thereof, and carries out friction stir welding with the center of the small diameter portion positioned to correspond to the joining surface. A joint portion of the welded members has a flat upper surface, and on both sides thereof are formed weld flashes B1. The weld flashes B1 are cut and removed to finish the joint portion. A connecting portion is connected via an arc R3 having a large curvature, and so stress concentration will not occur. With the plate thickness at the joint portion increased and the plate thickness of the flat panel portion minimized, the weight of the entire railway car structural body can be reduced.