while a lower plate 5 is made of carbon steel and has a thickness of 1 mm these test pieces are caused to lap each other and a welding tool 1 is pressed from a side of the upper plate 4 while rotating in addition the welding tool 1 comprises a shoulder 3 having a diameter of 12 mm and a tip end of the welding tool 1 defines a semispherical shaped portion 13 having a radius of curvature of 20 mm since the welding tool 1 is pressed frictional heat and pressure generated between the welding tool and the upper plate 4 cause the upper plate 1 to undergo plastic flow toward an outer peripheral side of the welding tool 1 to produce discharged metal portions 8 in the process of such discharge plastic flow is not generated in the lower plate since carbon steel forming the lower plate 2 is high in melting point however a surface oxide film on a welding boundary surface of carbon steel is removed by plastic flow of aluminum and so the welding boundary surfaces 6 are activated to enable the upper plate 1 and the lower plate 2 to be metallically welded furthermore since the welding tool 1 is pressed from a side of the upper plate 4 made of aluminum the welding temperature can be made low as compared with the case where welding is performed from a side of the lower plate 5 made of carbon steel therefore it is possible to obtain favorable welding characteristics for the welded boundary surfaces since generation of intermetallic compounds which reduce the fatigue strength.