A friction stir welding method for structural steel, the method welding steel sheets used as workpieces together by moving a pair of rotary tools (1,15) in a welding direction while rotating the pair of rotary tools (1,15) 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 pair of rotary tools (1,15) and the steel sheets and that the softened portion is stirred with the pair of rotary tools (1,15), the pair of rotary tools (1,15) each including a shoulder (8,16) and a pin (9,17) disposed on the shoulder (8,16) and sharing a rotation axis (2, 19) with the shoulder, at least the shoulder (8,16) and the pin (9,17) being made of a material harder than the steel sheets, the method comprising: arranging one of the pair of rotary tools (1,15) on one surface side and the other of the pair of rotary tools (1,15) on the other surface side of the steel sheets so as to face one another; holding the steel sheets using a holding device, causing each of the shoulders of the facing rotary tools to press the one surface side and the other surface side of the steel sheets, inserting each of the pins 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 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 T s, in °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 (x) between the rotary tool (1,15) and the heated region on the surface of the steel sheet is equal to or smaller than a diameter of the shoulder of the rotary tool (a), 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, and on the surface of the steel sheet, 50% or more of the area of the heated region is located between a welding centerline and a straight line that is parallel to the welding centerline and lies on an advancing side and at a distance equal to a maximum radius of the pin of the rotary tool from the welding centerline, the welding centerline being a straight line that passes through the rotation axis of the rotary tool (2,19) and is parallel to the welding direction, wherein a depth of a region that extends from the surface of the steel sheet in the heated region in a thickness direction and has a temperature T D, in °C, satisfying T D ≥ 0.8×T A1, where T A1 is described in Formula below, is defined as a depth of the heated region, wherein the depth of the heated region is 100% of a thickness (t) of the steel sheet, where Formula is T A1 (°C)=723-10.7[%Mn]-16.9[%Ni]+29.1[%Si]+16.9[%Cr]+290[%As]+6.38[%W] where [%M] represents a content, in mass%, of an M element in the steel sheet used as the workpiece, and if the M element is not contained, [%M] is 0. arranging one of the pair of rotary tools (1,15) on one surface side and the other of the pair of rotary tools (1,15) on the other surface side of the steel sheets so as to face one another; holding the steel sheets using a holding device, causing each of the shoulders of the facing rotary tools to press the one surface side and the other surface side of the steel sheets, inserting each of the pins 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 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 T s, in °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 (x) between the rotary tool (1,15) and the heated region on the surface of the steel sheet is equal to or smaller than a diameter of the shoulder of the rotary tool (a), 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, and on the surface of the steel sheet, 50% or more of the area of the heated region is located between a welding centerline and a straight line that is parallel to the welding centerline and lies on an advancing side and at a distance equal to a maximum radius of the pin of the rotary tool from the welding centerline, the welding centerline being a straight line that passes through the rotation axis of the rotary tool (2,19) and is parallel to the welding direction, wherein a depth of a region that extends from the surface of the steel sheet in the heated region in a thickness direction and has a temperature T D, in °C, satisfying T D ≥ 0.8×T A1, where T A1 is described in Formula below, is defined as a depth of the heated region, wherein the depth of the heated region is 100% of a thickness (t) of the steel sheet, where Formula is T A1 (°C)=723-10.7[%Mn]-16.9[%Ni]+29.1[%Si]+16.9[%Cr]+290[%As]+6.38[%W] where [%M] represents a content, in mass%, of an M element in the steel sheet used as the workpiece, and if the M element is not contained, [%M] is 0.