The purpose of the present invention is to perform, in a preferable manner, friction stir welding of a part of a metal material to be welded by minimizing the occurrence of welding defects while minimizing the load applied to a rotary tool even when there is change in the thickness of the part to be welded. The present invention is provided with: a first friction stir welding step for forming a first stirred region (E1) in a groove part (6); and a second friction stir welding step for moving, after or simultaneously with the first friction stir welding step, a second rotary tool arranged on the lower side in the thickness direction with respect to the groove part (6) while rotating the second rotary tool, thereby a second stirred region (E2) is formed in the groove part (6). The first stirred region (E1) reaches the interior of the groove part (6) from the upper side of the groove part (6), and the second stirred region (E2) reaches the interior from the lower side of the groove part (6). The first stirred re
