A friction stir welding method for a metal material for longitudinally welding members to be butted and then welded with complicated shape portions in section at end portions. The friction stir welding method includes preparing members to be welded and formed with excess thickness portions and a welding tool provided with a probe; performing a first welding to the members to be welded; inserting the welding tool from the excess thickness portion to cause plastic flow of the metal material subsequent to the butt-welding; and performing a second welding.