A friction stir welding method comprising: inserting a rotating tool into an unwelded portion (13,20) where two or more steel sheets (21,22) are overlapped or butted together; moving the rotating tool along portions to be welded while rotating the tool, so that a softened portion is formed in the steel sheets (21,22) by friction heat generated between the rotating tool and the steel sheets (21,22), and the steel sheets (21,22) are welded together by utilizing a plastic flow generated by the softened portion being stirred; and preheating the unwelded portion (13,20) before welding thereof by the rotating tool using a heating device disposed ahead of the rotating tool in the advancing direction, characterized in that a surface temperature distribution in a direction perpendicular to an advancing direction in the steel sheets (21,22) is caused by the preheating, the surface temperature distribution at a position where welding is performed by the rotating tool being such that a maximum temperature T U satisfies the range of 0.6 × T Ac1 < T U < 1.8 × T Ac1, and a width L of a range exceeding a temperature T L = 0.6 × T Ac1 satisfies a relation of 0.3 × d ≤ L ≤ 2.0 × d with respect to a diameter d of a shoulder of the rotating tool, and wherein a rear heating device for re-heating the welded portion (14,25) is disposed behind the rotating tool in the advancing direction, and the maximum temperature T p after re-heating of the region to be re-heated by the rear heating device satisfies the relation of 0.6 × T Ac1 ≤ T p ≤ 1.2×T Ac1: where T Ac1 is a temperature defined by the following formula using amounts of added elements of the steel sheets (21,22): T Ac 1 = 723 − 10.7 % Mn − 16.9 % Ni + 29.1 % Si + 16.9 % Cr + 290 % As + 6.38 % W where [%M] represents the content of M element (mass%) in the steel sheets (21,22) inserting a rotating tool into an unwelded portion (13,20) where two or more steel sheets (21,22) are overlapped or butted together; moving the rotating tool along portions to be welded while rotating the tool, so that a softened portion is formed in the steel sheets (21,22) by friction heat generated between the rotating tool and the steel sheets (21,22), and the steel sheets (21,22) are welded together by utilizing a plastic flow generated by the softened portion being stirred; and preheating the unwelded portion (13,20) before welding thereof by the rotating tool using a heating device disposed ahead of the rotating tool in the advancing direction, characterized in that a surface temperature distribution in a direction perpendicular to an advancing direction in the steel sheets (21,22) is caused by the preheating, the surface temperature distribution at a position where welding is performed by the rotating tool being such that a maximum temperature T U satisfies the range of 0.6 × T Ac1 < T U < 1.8 × T Ac1, and a width L of a range exceeding a temperature T L = 0.6 × T Ac1 satisfies a relation of 0.3 × d ≤ L ≤ 2.0 × d with respect to a diameter d of a shoulder of the rotating tool, and wherein a rear heating device for re-heating the welded portion (14,25) is disposed behind the rotating tool in the advancing direction, and the maximum temperature T p after re-heating of the region to be re-heated by the rear heating device satisfies the relation of 0.6 × T Ac1 ≤ T p ≤ 1.2×T Ac1: where T Ac1 is a temperature defined by the following formula using amounts of added elements of the steel sheets (21,22): T Ac 1 = 723 − 10.7 % Mn − 16.9 % Ni + 29.1 % Si + 16.9 % Cr + 290 % As + 6.38 % W where [%M] represents the content of M element (mass%) in the steel sheets (21,22)