The invention provides a method for friction stir welding a stainless steel panel to an extruded aluminum alloy shape member. A side structure (2) of a car body (1) is formed by friction stir welding aluminum alloy extruded hollow shape members (30 and 40) from the inner side of the car. On the outer side of the side structure (2) are stainless steel panels (10 and 20) friction stir welded to the butted portion between hollow shape members (30 and 40) from the inner side of the car. After friction stir welding the butted portion between hollow shape members (30 and 40), inner face plates (31 and 41) of the hollow shape members (30 and 40) are connected via a connecting member (51). According to the present method, scratches on the surface of the car body can be reduced by arranging stainless steel panels (10 and 20) on the outer side of the car. It is possible to arrange heat insulation members between the stainless steel panels and the hollow shape members.