A continuous welding process known as friction stir welding (“FSW” below) has been applied to the welding of steel and aluminum die-cast members, and has been employed in mass production for the first time. The use of this new welding process has made it possible to realize a weight saving of 25% in the subframe against the previous model, and the stiffness of the installation points in the suspension system has been increased by 20%. This has contributed to the achievement of enhanced maneuverability. In order to apply this welding technique to mass production, a unique welding system employing a general-purpose multi-joint robot was developed, and a level of manufacturing efficiency and infrastructure investment cost equivalent to conventional MIG welding were realized. A non-destructive inspection system using a high-sensitivity infrared camera and a laser beam has also been developed in order to verify the strength of the FSW joints.