a friction stirring - welding method capable of increasing the welding strength of a lap joint . a welding tool comprises a small diameter projected part at the tip of a shoulder . the welding tool is press - fitted to the upper plate of the lap joint while being rotated . by a friction stirring action , an upper plate side welding boundary surface plastically flows , a surface oxidation film on a lapped surface is peeled off , and the boundary surface is activated to provide an excellent welding part . also , since the welding tool is formed in a shape having the small diameter semispherical projected part at the tip thereof , the thickness of an upper plate welding part is increased , and a lap joint part with high welding strength can be provided .