A method of friction stir welding, using a friction stir welding tool having a first shoulder surface configured to contact a front surface of a subject work piece; a second shoulder surface disposed so as to face the first shoulder surface and being configured to contact a rear surface of the work piece; a shaft portion connecting the first shoulder surface and the second shoulder surface to each other with a fixed gap therebetween; and a vortex groove (33, 36) disposed in at least one of the first shoulder surface and the second shoulder surface, wherein the vortex groove (33, 36) extends in a predetermined tool rotation direction so as to extend toward an outer peripheral side in the first shoulder surface or the second shoulder surface and has an opening at an outer peripheral side edge of the first shoulder surface or the second shoulder surface, and the friction stir welding tool being capable of rotating about an axis line of the shaft portion in the predetermined tool rotation direction so as to perform friction stir welding on the work piece, the method characterized in that the depth of the vortex groove (33, 36) of the friction stir welding tool is constant throughout the entire area in the extension direction of the vortex groove (33, 36), and by comprising the steps of: determining a minimum allowable thickness (M LMS) of the work piece for the friction stir welding tool; and friction stir welding the work piece having at least such minimal allowable thickness with the friction stir welding tool, wherein the minimum allowable thickness (M LMS) of the work piece is determined by the following equation, W LMS > = T 1 + ΔL where: W LMS is a minimum allowable thickness of the work piece; T1 is the gap between the first shoulder surface and the second shoulder surface in the axial direction of the shaft portion; and ΔL is the thermal expansion amount in the axial direction of the shaft portion during friction stir welding. a first shoulder surface configured to contact a front surface of a subject work piece; a second shoulder surface disposed so as to face the first shoulder surface and being configured to contact a rear surface of the work piece; a shaft portion connecting the first shoulder surface and the second shoulder surface to each other with a fixed gap therebetween; and a vortex groove (33, 36) disposed in at least one of the first shoulder surface and the second shoulder surface, wherein the vortex groove (33, 36) extends in a predetermined tool rotation direction so as to extend toward an outer peripheral side in the first shoulder surface or the second shoulder surface and has an opening at an outer peripheral side edge of the first shoulder surface or the second shoulder surface, and the friction stir welding tool being capable of rotating about an axis line of the shaft portion in the predetermined tool rotation direction so as to perform friction stir welding on the work piece, the method characterized in that the depth of the vortex groove (33, 36) of the friction stir welding tool is constant throughout the entire area in the extension direction of the vortex groove (33, 36), and by comprising the steps of: determining a minimum allowable thickness (M LMS) of the work piece for the friction stir welding tool; and friction stir welding the work piece having at least such minimal allowable thickness with the friction stir welding tool, wherein the minimum allowable thickness (M LMS) of the work piece is determined by the following equation, W LMS > = T 1 + ΔL where: W LMS is a minimum allowable thickness of the work piece; T1 is the gap between the first shoulder surface and the second shoulder surface in the axial direction of the shaft portion; and ΔL is the thermal expansion amount in the axial direction of the shaft portion during friction stir welding. determining a minimum allowable thickness (M LMS) of the work piece for the friction stir welding tool; and friction stir welding the work piece having at least such minimal allowable thickness with the friction stir welding tool, wherein W LMS is a minimum allowable thickness of the work piece; T1 is the gap between the first shoulder surface and the second shoulder surface in the axial direction of the shaft portion; and ΔL is the thermal expansion amount in the axial direction of the shaft portion during friction stir welding.