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 .