A double-side friction stir welding method in which a pair of rotating tools (1, 8) facing each other is respectively disposed on a top-surface side and a bottom-surface side of a butted portion or an overlapping portion, which comprises a joint portion of two metal sheets or two metal plates; the pair of rotating tools (1, 8) are moved in a welding direction while being rotated at the butted portion or the overlapping portion; and while the metal sheets or the metal plates are softened by frictional heat generated between the rotating tools (1, 8) and the metal sheets or the metal plates softened parts are stirred by the rotating tools (1, 8) to produce a plastic flow to join the metal sheets or the metal plates to each other, wherein each of the pair of rotating tools (1, 8) includes a shoulder part (5, 9) and a pin part (6, 10) that is disposed at the shoulder part (5, 9) and that along with the shoulder part (5, 9) includes a common rotation axis (2, 11), and at least the shoulder parts (5, 9) and the pin parts (6, 10) are made of materials that are harder than materials of the metal sheets or the metal plates, while the metal sheets or the metal plates are fixed by a gripping device, the pair of rotating tools (1, 8) is respectively pushed against a top surface and a bottom surface of each metal sheet or metal plate, and the rotating tools (1, 8) are moved in the welding direction while being rotated, and the rotation axes (2, 11) of the pair of respective rotating tools (1, 8) are inclined at an inclination angle α (°) on a preceding side with respect to the welding direction from a vertical direction with respect to the metal sheets or the metal plates, the inclination angle α satisfies 0 ≤ α ≤ 3, and the pair of rotating tools (1, 8) is further rotated in opposite directions to perform the friction stir welding, characterized in that a gap G (mm) between the shoulder parts (5, 9) that is formed by providing a gap g (mm) between ends of the pin parts (6, 10) of the pair of respective rotating tools (1, 8) with respect to a thickness t (mm) of each metal sheet or metal plate, or with respect to a total thickness t (mm) of the overlapped metal sheets or the overlapped metal plates, and a diameter D (mm) of the shoulder part (5, 9) of each rotating tool (1, 8) satisfies 0.6 × t − 0.2 × D × sin α ≤ G ≤ 0.9 × t − 0.2 × D × sin α. while being rotated at the butted portion or the overlapping portion; and while the metal sheets or the metal plates are softened by frictional heat generated between the rotating tools (1, 8) and the metal sheets or the metal plates softened parts are stirred by the rotating tools (1, 8) to produce a plastic flow to join the metal sheets or the metal plates to each other, wherein each of the pair of rotating tools (1, 8) includes a shoulder part (5, 9) and a pin part (6, 10) that is disposed at the shoulder part (5, 9) and that along with the shoulder part (5, 9) includes a common rotation axis (2, 11), and at least the shoulder parts (5, 9) and the pin parts (6, 10) are made of materials that are harder than materials of the metal sheets or the metal plates, while the metal sheets or the metal plates are fixed by a gripping device, the pair of rotating tools (1, 8) is respectively pushed against a top surface and a bottom surface of each metal sheet or metal plate, and the rotating tools (1, 8) are moved in the welding direction while being rotated, and the rotation axes (2, 11) of the pair of respective rotating tools (1, 8) are inclined at an inclination angle α (°) on a preceding side with respect to the welding direction from a vertical direction with respect to the metal sheets or the metal plates, the inclination angle α satisfies 0 ≤ α ≤ 3, and the pair of rotating tools (1, 8) is further rotated in opposite directions to perform the friction stir welding, a gap G (mm) between the shoulder parts (5, 9) that is formed by providing a gap g (mm) between ends of the pin parts (6, 10) of the pair of respective rotating tools (1, 8) with respect to a thickness t (mm) of each metal sheet or metal plate, or with respect to a total thickness t (mm) of the overlapped metal sheets or the overlapped metal plates, and a diameter D (mm) of the shoulder part (5, 9) of each rotating tool (1, 8) satisfies 0.6 × t − 0.2 × D × sin α ≤ G ≤ 0.9 × t − 0.2 × D × sin α.