A friction stir welding method for structural steel, the method welding steel sheets used as workpieces together by moving a rotary tool in a welding direction while rotating the rotary tool in a non-welded part of the steel sheets, and thus by producing plastic flow as a result of that the steel sheets are softened by frictional heat generated between the rotary tool and the steel sheets and that the softened portion is stirred with the rotary tool, the rotary tool including a shoulder and a pin disposed on the shoulder and sharing a rotation axis with the shoulder, at least the shoulder and the pin being made of a material harder than the steel sheets, the method comprising: heating at least one of the steel sheets using heating means provided in front of the rotary tool on the one surface side that moves in the welding direction, wherein, a region of a surface of the steel sheet whose surface temperature Ts(°C) is increased to T s ≥ 0.8×T A1 where T A1 is described in Formula below by the heating is defined as a heated region, a minimum distance between the rotary tool and the heated region on the surface of the steel sheet is equal to or smaller than a diameter (a) of the shoulder of the rotary tool, and an area of the heated region on the surface of the steel sheet is equal to or smaller than an area of a maximum diameter part of the pin of the rotary tool, T A 1 ° C = 723 − 10.7 % Mn − 16.9 % Ni + 29.1 % Si + 16.9 % Cr + 290 % As + 6.38 % W where [%M] represents a content (mass%) of an M element in the steel sheet used as the workpiece, and if the M element is not contained, [%M] is 0; characterized by: arranging a said rotary tool on one surface side of the steel sheets and another said rotary tool on the other surface side of the steel sheets so that the rotary tools (1, 15) face one another; holding the steel sheets using a holding device, causing the respective shoulders (8, 16) of the facing rotary tools (1, 15) to press the one surface side and the other surface side of the steel sheets, inserting the respective pins (9, 17) of the facing rotary tools (1, 15) into the non-welded part of the steel sheets from the one surface side and the other surface side, and moving the rotary tools (1, 15) in the welding direction while rotating the rotary tools, and wherein a depth D of a region that extends from the surface of the steel sheet in the heated region in a thickness direction is 100% of a thickness t of the steel sheet, said region having a temperature T D (°C) satisfying T D ≥ 0.8×T A1 where T A1 is described in Formula above. heating at least one of the steel sheets using heating means provided in front of the rotary tool on the one surface side that moves in the welding direction, wherein, a region of a surface of the steel sheet whose surface temperature Ts(°C) is increased to T s ≥ 0.8×T A1 where T A1 is described in Formula below by the heating is defined as a heated region, a minimum distance between the rotary tool and the heated region on the surface of the steel sheet is equal to or smaller than a diameter (a) of the shoulder of the rotary tool, and an area of the heated region on the surface of the steel sheet is equal to or smaller than an area of a maximum diameter part of the pin of the rotary tool, T A 1 ° C = 723 − 10.7 % Mn − 16.9 % Ni + 29.1 % Si + 16.9 % Cr + 290 % As + 6.38 % W where [%M] represents a content (mass%) of an M element in the steel sheet used as the workpiece, and if the M element is not contained, [%M] is 0; characterized by: arranging a said rotary tool on one surface side of the steel sheets and another said rotary tool on the other surface side of the steel sheets so that the rotary tools (1, 15) face one another; holding the steel sheets using a holding device, causing the respective shoulders (8, 16) of the facing rotary tools (1, 15) to press the one surface side and the other surface side of the steel sheets, inserting the respective pins (9, 17) of the facing rotary tools (1, 15) into the non-welded part of the steel sheets from the one surface side and the other surface side, and moving the rotary tools (1, 15) in the welding direction while rotating the rotary tools, and wherein a depth D of a region that extends from the surface of the steel sheet in the heated region in a thickness direction is 100% of a thickness t of the steel sheet, said region having a temperature T D (°C) satisfying T D ≥ 0.8×T A1 where T A1 is described in Formula above. arranging a said rotary tool on one surface side of the steel sheets and another said rotary tool on the other surface side of the steel sheets so that the rotary tools (1, 15) face one another; holding the steel sheets using a holding device, causing the respective shoulders (8, 16) of the facing rotary tools (1, 15) to press the one surface side and the other surface side of the steel sheets, inserting the respective pins (9, 17) of the facing rotary tools (1, 15) into the non-welded part of the steel sheets from the one surface side and the other surface side, and moving the rotary tools (1, 15) in the welding direction while rotating the rotary tools, and wherein a depth D of a region that extends from the surface of the steel sheet in the heated region in a thickness direction is 100% of a thickness t of the steel sheet, said region having a temperature T D (°C) satisfying T D ≥ 0.8×T A1 where T A1 is described in Formula above.