A friction stir welding method comprising: inserting a rotational tool into a weld portion of steel sheets or plates, the rotational tool comprising a shoulder and a pin disposed on the shoulder and sharing an axis of rotation with the shoulder, at least the shoulder and pin being made of a material harder than the steel sheets or plates as working materials; and moving the rotational tool while rotating the tool, so that the steel sheets or plates are softened by frictional heat generated between the rotational tool and the steel sheets or plates, and a plastic flow is generated by the softened part being stirred by the rotational tool, and the steel sheets or plates are welded, wherein the steel sheets or plates are each made of high-strength welding structural steel having a chemical composition consisting of by mass% C: 0.03 % to 0.12 %, Si: 0.6 % or less, Mn: 1.5 % to 3.0 %, P: 0.015 % or less, S: 0.002 % or less, Al: 0.1 % or less, Ti: 0.005 % to 0.030 %, Nb: 0.01 % to 0.10 %, N: 0.001 % to 0.008%, O: 0.03 % or less, and optionally further one or more elements selected from Cu: 1.0 % or less, Ni: 1.5 % or less, Mo: 1.0 % or less, Cr: 1.0 % or less, V: 0.10 % or less, W: 0.2 % to 1.2 %, B: 0.0001 % to 0.005 %, Ca: 0.01 % or less, REM: 0.02 % or less, Mg: 0.01 % or less, and Zr: 0.0005 % to 0.03 %, a Pcm value calculated by the following formula satisfying 0.18 ≤ Pcm ≤ 0.30, and the balance being Fe and incidental impurities, friction stir welding conditions fall under ranges of tool rotational speed: 100 rpm to 1000 rpm, tool rotational torque: 50 N·m to 500 N·m, and welding speed: 10 mm/min to 1000 mm/min, and a welding heat input HI defined by the following formula falls under a range of 1.5 to 20, and satisfies a range of the following formula in relation with Pcm, Pcm % = C + Si / 30 + Mn + Cr + Cu / 20 + Ni / 60 + Mo / 15 + V / 10 + 5 B HI kJ / mm = 6.28 × RT × RS / TS / 1000 where RT represents tool rotational torque (N·m), RS represents tool rotational speed (rpm), and TS represents welding speed (mm/min), 1.5 × 10 9 × Pcm 13.8 ≤ HI ≤ 2.1 × 10 8 × Pcm 10.6 inserting a rotational tool into a weld portion of steel sheets or plates, the rotational tool comprising a shoulder and a pin disposed on the shoulder and sharing an axis of rotation with the shoulder, at least the shoulder and pin being made of a material harder than the steel sheets or plates as working materials; and moving the rotational tool while rotating the tool, so that the steel sheets or plates are softened by frictional heat generated between the rotational tool and the steel sheets or plates, and a plastic flow is generated by the softened part being stirred by the rotational tool, and the steel sheets or plates are welded, wherein the steel sheets or plates are each made of high-strength welding structural steel having a chemical composition consisting of by mass% C: 0.03 % to 0.12 %, Si: 0.6 % or less, Mn: 1.5 % to 3.0 %, P: 0.015 % or less, S: 0.002 % or less, Al: 0.1 % or less, Ti: 0.005 % to 0.030 %, Nb: 0.01 % to 0.10 %, N: 0.001 % to 0.008%, O: 0.03 % or less, and optionally further one or more elements selected from Cu: 1.0 % or less, Ni: 1.5 % or less, Mo: 1.0 % or less, Cr: 1.0 % or less, V: 0.10 % or less, W: 0.2 % to 1.2 %, B: 0.0001 % to 0.005 %, Ca: 0.01 % or less, REM: 0.02 % or less, Mg: 0.01 % or less, and Zr: 0.0005 % to 0.03 %, a Pcm value calculated by the following formula satisfying 0.18 ≤ Pcm ≤ 0.30, and the balance being Fe and incidental impurities, C: 0.03 % to 0.12 %, Si: 0.6 % or less, Mn: 1.5 % to 3.0 %, P: 0.015 % or less, S: 0.002 % or less, Al: 0.1 % or less, Ti: 0.005 % to 0.030 %, Nb: 0.01 % to 0.10 %, N: 0.001 % to 0.008%, O: 0.03 % or less, and optionally further one or more elements selected from Cu: 1.0 % or less, Ni: 1.5 % or less, Mo: 1.0 % or less, Cr: 1.0 % or less, V: 0.10 % or less, W: 0.2 % to 1.2 %, B: 0.0001 % to 0.005 %, Ca: 0.01 % or less, REM: 0.02 % or less, Mg: 0.01 % or less, and Zr: 0.0005 % to 0.03 %, a Pcm value calculated by the following formula satisfying 0.18 ≤ Pcm ≤ 0.30, and the balance being Fe and incidental impurities, friction stir welding conditions fall under ranges of tool rotational speed: 100 rpm to 1000 rpm, tool rotational torque: 50 N·m to 500 N·m, and welding speed: 10 mm/min to 1000 mm/min, and tool rotational speed: 100 rpm to 1000 rpm, tool rotational torque: 50 N·m to 500 N·m, and welding speed: 10 mm/min to 1000 mm/min, and a welding heat input HI defined by the following formula falls under a range of 1.5 to 20, and satisfies a range of the following formula in relation with Pcm, Pcm % = C + Si / 30 + Mn + Cr + Cu / 20 + Ni / 60 + Mo / 15 + V / 10 + 5 B HI kJ / mm = 6.28 × RT × RS / TS / 1000 where RT represents tool rotational torque (N·m), RS represents tool rotational speed (rpm), and TS represents welding speed (mm/min), 1.5 × 10 9 × Pcm 13.8 ≤ HI ≤ 2.1 × 10 8 × Pcm 10.6