To provide a method of friction stir welding which can prevent end faces of the joining parts from separating at the butt without providing rib, etc., even in case that the backing material cannot be utilized.

In the method of joining two parts by friction stir welding consisting of forming an engagement fitting each other at the butt 10 of both parts, butting both parts by engaging at the butt, pressing a tool 9 against the butt 10 under rotation and joining both parts by friction stir welding, the engagement has a configuration where both parts cannot be rotated around the butt 10. For another method of joining two parts by friction stir welding consisting of forming an engagement fitting each other at the butt 10 of both parts, butting both parts by engaging at the butt, pressing a tool 9 against the butt 10 under rotation and joining both parts by friction stir welding, the engagement has a configuration which can allow engaging of both parts only by inserting each part into another in the direction parallel to the surface of joining parts.