To provide a welding method where, upon the friction stir welding of metallic members having different melting points each other, a sound welded part free from defects at the inside and in the vicinity of the surface can be securely obtained, and to provide a welding tool used therefor.

The welding tool 20' used for friction stir welding comprises: a first part 20a including a columnar body 21; a second part 20b including a rotary axis 26 passing through the central part of the body 21 at the coaxial center and a prove 25 located at the tip of the rotary axis 26 and projecting from the bottom face 21a of the body 21; and a bearing 28 arranged between the first part 20a and the second part 20b.