A friction stir welding method is provided that can prevent defective welding due to a shortage of metal. The friction stir welding method welds metal members (1, 2) using a primary joining rotary tool (F) having a stirring pin (F2), and includes steps of: butting in which the metal members (1, 2) are butted with each other at an angle to form a butted portion (J1); buildup welding in which buildup welding is applied along an inner corner of the metal members (1, 2) formed in the butting step to cover the inner corner by a weld metal (M); and inner corner joining in which only the stirring pin (F2) in rotation is inserted in the inner corner to plastically fluidize the weld metal (M) and the metal members (1, 2) for friction stir welding of the butted portion (J1).