The invention provides a friction stir welding method of forming a weld portion having an excellent fatigue property in the case of welding a weld material by using a friction stir welding method. The invention provides a friction stir welding method structured such as to insert a tool provided in such a manner that a pin-shaped probe protrudes to an end surface of a columnar member to a weld material in a state of being rotated, and move the tool in a state of being rotated, thereby welding two or more weld materials in accordance with a friction stirring, in which a cavity portion is provided in a part of a weld portion of the weld material, a member is formed in the cavity portion, and the weld material is welded by the friction stirring. In accordance with the invention, since an unwelded portion is not formed near the cavity portion, a notch effect is reduced, and it is possible to form the weld member which is excellent in the fatigue property.