A both-side friction stir welding method for joining two metal plates (1, 2) together, in which upper and lower rotary tools (5, 6) each including a tool body (5a, 5b), a probe (13a, 13b) formed at a distal end of the tool body (5a, 5b) and having a diameter smaller than that of the distal end of the tool body and a shoulder (5b, 6b) formed around a portion of the distal end of the tool body at which the probe is formed are inserted into a butting or lapping portion formed as a welding portion between the two metal plates, from a front surface side and a back surface side of the butting portion, respectively, while the upper and lower rotary tools are rotated to frictionally stir the butting portion by utilizing frictional heat generated by the rotation of the tools thereby to weld the two metal plates; characterized in that the method comprises: gripping front and back surfaces of the metal plates (1, 2) with first and second gripping devices (7, 8), respectively, the metal plates (1, 2) each having a thickness smaller than a diameter of each of the shoulders (5b, 6b) of the upper and lower rotary tools (5, 6), the first and second gripping devices (7, 8) being arranged to sandwich the upper and lower rotary tools (5, 6) in the opposed sides thereof and spaced apart from each other at a distance not less than 1.5 times, but not greater than 5.0 times the diameters of the tool bodies (5a, 5b); disposing the upper and lower rotary tools in opposed relationship in the front surface side and the back surface side of the butting or lapping portion of the two metal plates, respectively, moving the upper and lower rotary tools in a direction in which the upper and lower rotary tools approach to each other, and imparting a predetermined clearance (δ) set to be a ratio not less than 1%, but not greater than 50% of the thickness of the welding portion of the two metal plates (1, 2), between distal ends of the probes (13a, 13b) of the upper and lower rotary tools while pressing the shoulders (5b, 6b) thereof against the front surface side and the back surface side of the butting or lapping portion; and moving, under that state, the upper and lower rotary tools along the butting or lapping portion while rotating the rotary tools thereby to perform the friction stir welding. gripping front and back surfaces of the metal plates (1, 2) with first and second gripping devices (7, 8), respectively, the metal plates (1, 2) each having a thickness smaller than a diameter of each of the shoulders (5b, 6b) of the upper and lower rotary tools (5, 6), the first and second gripping devices (7, 8) being arranged to sandwich the upper and lower rotary tools (5, 6) in the opposed sides thereof and spaced apart from each other at a distance not less than 1.5 times, but not greater than 5.0 times the diameters of the tool bodies (5a, 5b); disposing the upper and lower rotary tools in opposed relationship in the front surface side and the back surface side of the butting or lapping portion of the two metal plates, respectively, moving the upper and lower rotary tools in a direction in which the upper and lower rotary tools approach to each other, and imparting a predetermined clearance (δ) set to be a ratio not less than 1%, but not greater than 50% of the thickness of the welding portion of the two metal plates (1, 2), between distal ends of the probes (13a, 13b) of the upper and lower rotary tools while pressing the shoulders (5b, 6b) thereof against the front surface side and the back surface side of the butting or lapping portion; and moving, under that state, the upper and lower rotary tools along the butting or lapping portion while rotating the rotary tools thereby to perform the friction stir welding.