A friction stir welding method for joining two metal members (1, 2, 10, 20, 40, 101, 102) by a rotary tool having a stirring pin, comprising steps of: butting in which the metal members (1, 2, 10, 20, 40, 101, 102) are butted with each other to form a butted portion (J1, J2) having an inner corner (P1, P2) and an outer corner; auxiliary member arranging in which an auxiliary member (7, 8, 9, 30, 31, 50) is arranged along the inner corner (P1, P2) of the metal members (1, 2, 10, 20, 40, 101, 102) formed in the butting step; and inner corner joining in which only the stirring pin of the rotary tool in rotation is inserted in the auxiliary member (7, 8, 9, 30, 31, 50) and the inner corner (P1, P2) to plastically fluidize the auxiliary member (7, 8, 9, 30, 31, 50) and the metal members (1, 2, 10, 20, 40, 101, 102) for friction stir welding of the butted portion (J1, J2), characterized by the stirring pin having a spiral groove (F3, G3, H3) formed on an outer circumferential face of the stirring pin, wherein in a case where a spiral groove (F3, G3, H3) is formed counterclockwise from a base end of the stirring pin toward a tip end of the stirring pin, the rotary tool is rotated clockwise, and in a case where a spiral groove (F3, G3, H3) is formed clockwise from the base end of the stirring pin toward the tip end of the stirring pin, the rotary tool is rotated counterclockwise, wherein in the inner corner joining step, only the stirring pin is inserted into the butted portion (J1, J2) via the auxiliary member (7, 8, 9, 30, 31, 50) in a state where a coupling portion of the rotary tool is separated from both of the metal members (1, 2, 10, 20, 40, 101, 102) and the auxiliary member (7, 8, 9, 30, 31, 50), wherein the coupling portion is coupled to a rotary shaft of a friction stirring apparatus, and wherein the stirring pin extends downward from and coaxial with the coupling portion and tapers off with increasing distance from the coupling portion. butting in which the metal members (1, 2, 10, 20, 40, 101, 102) are butted with each other to form a butted portion (J1, J2) having an inner corner (P1, P2) and an outer corner; auxiliary member arranging in which an auxiliary member (7, 8, 9, 30, 31, 50) is arranged along the inner corner (P1, P2) of the metal members (1, 2, 10, 20, 40, 101, 102) formed in the butting step; and inner corner joining in which only the stirring pin of the rotary tool in rotation is inserted in the auxiliary member (7, 8, 9, 30, 31, 50) and the inner corner (P1, P2) to plastically fluidize the auxiliary member (7, 8, 9, 30, 31, 50) and the metal members (1, 2, 10, 20, 40, 101, 102) for friction stir welding of the butted portion (J1, J2), characterized by the stirring pin having a spiral groove (F3, G3, H3) formed on an outer circumferential face of the stirring pin, wherein in a case where a spiral groove (F3, G3, H3) is formed counterclockwise from a base end of the stirring pin toward a tip end of the stirring pin, the rotary tool is rotated clockwise, and in a case where a spiral groove (F3, G3, H3) is formed clockwise from the base end of the stirring pin toward the tip end of the stirring pin, the rotary tool is rotated counterclockwise, wherein in the inner corner joining step, only the stirring pin is inserted into the butted portion (J1, J2) via the auxiliary member (7, 8, 9, 30, 31, 50) in a state where a coupling portion of the rotary tool is separated from both of the metal members (1, 2, 10, 20, 40, 101, 102) and the auxiliary member (7, 8, 9, 30, 31, 50), wherein the coupling portion is coupled to a rotary shaft of a friction stirring apparatus, and wherein the stirring pin extends downward from and coaxial with the coupling portion and tapers off with increasing distance from the coupling portion.