This paper describes an application of friction stir welding (FSW), solid phase welding, to fabricate railway car body made of aluminium hollow extrusions (double-skinned car body). The construction of double-skinned car body using FSW can reduce deformation and release process from skilled workers compared with the conventional method. In this paper, from the viewpoint of strength, we show our approach from a design of the joint's sectional shape to the final evaluation test using actual double-skinned car body fabricated by FSW. For design, reaction force in the workpiece coincident with heat input was considered. At load test, we evaluated stress levels at the joint, accuracy of the simulation of the car body and double-skinned car body's characteristics. As a result, railway car body joined by FSW can meet all the requirements for actual application. These benefits have been applied to 319 vehicles as of June 2002.