A double-sided friction stir welding method comprising: disposing a pair of opposed rotating tools (1, 8) on top and bottom sides of a butt joint between two metal sheets; moving the pair of rotating tools (1, 8) along the butt joint in a welding direction while the pair of rotating tools (1, 8) are rotated to thereby soften a portion of the metal sheets by heat of friction between the rotating tools (1, 8) and the metal sheets; and stirring the softened portion with the rotating tools (1, 8) to generate plastic flow to thereby join the metal sheets together, wherein each of the pair of rotating tools (1, 8) used includes a shoulder (5, 9) and a pin (6, 10) that is disposed on the shoulder (5, 9) and shares a rotation axis (2, 11) with the shoulder (5, 9), at least the shoulder (5, 9) and the pin (6, 10) being formed of a material harder than the metal sheets, wherein, with the metal sheets fixed by a holding unit, the pair of rotating tools (1, 8) are pressed against the top and bottom sides of the metal sheets and moved in the welding direction while rotated, wherein the rotation axes of the pair of rotating tools (1, 8) are tilted at a tilt angle α (°) with respect to a direction normal to the metal sheets such that tips of the pins (6, 10) are located on a leading side in the welding direction, and the tilt angle α satisfies 0 < α ≤ 3, characterized in that a gap G (mm) between the shoulders (5, 9) that is created by forming a gap g (mm) between the tips of the pins (6, 10) of the pair of rotating tools (1, 8) satisfies 0.5 × t − 0.2 × D × sin α ≤ G ≤ t − 0.2 × D × sin α, where t is the thickness (mm) of each of the metal sheets, and D is the diameter (mm) of the shoulders (5, 9) of the rotating tools (1, 8), wherein the diameter D (mm) of the shoulders (5, 9) and the thickness t (mm) of each of the metal sheets satisfy 4 × t ≤ D ≤ 20 × t, wherein the gap g, the thickness t (mm) of each of the metal sheets, and the diameter D (mm) of the shoulders (5, 9) of the rotating tools (1, 8) satisfy 0.1 − 0.09 × exp − 0.011 × D / t 2 × t ≤ g ≤ 1 − 0.09 × exp − 0.011 × D / t 2 × t, wherein the pair of rotating tools (1, 8) are rotated in opposite directions (Q, R) to perform friction stir welding, and wherein the numbers of revolutions S (rpm) of the pair of rotating tools (1, 8) rotated in the opposite directions (Q, R) are the same, and the ratio T/S of a welding speed T (m/min) of the rotating tools (1, 8) to the number of revolutions S of the rotating tools (1, 8), the gap G (mm) between the shoulders (5, 9), the diameter D (mm) of the shoulders (5, 9), and the thickness t (mm) of each of the metal sheets satisfy T/S ≤ 1 / 1000 × D/t × 34.5 − 32.2 × G/t / 53 − 3.4 × D/t. wherein each of the pair of rotating tools (1, 8) used includes a shoulder (5, 9) and a pin (6, 10) that is disposed on the shoulder (5, 9) and shares a rotation axis (2, 11) with the shoulder (5, 9), at least the shoulder (5, 9) and the pin (6, 10) being formed of a material harder than the metal sheets, wherein, with the metal sheets fixed by a holding unit, the pair of rotating tools (1, 8) are pressed against the top and bottom sides of the metal sheets and moved in the welding direction while rotated, wherein the rotation axes of the pair of rotating tools (1, 8) are tilted at a tilt angle α (°) with respect to a direction normal to the metal sheets such that tips of the pins (6, 10) are located on a leading side in the welding direction, and the tilt angle α satisfies 0 < α ≤ 3, characterized in that a gap G (mm) between the shoulders (5, 9) that is created by forming a gap g (mm) between the tips of the pins (6, 10) of the pair of rotating tools (1, 8) satisfies 0.5 × t − 0.2 × D × sin α ≤ G ≤ t − 0.2 × D × sin α, where t is the thickness (mm) of each of the metal sheets, and D is the diameter (mm) of the shoulders (5, 9) of the rotating tools (1, 8), wherein the diameter D (mm) of the shoulders (5, 9) and the thickness t (mm) of each of the metal sheets satisfy 4 × t ≤ D ≤ 20 × t, wherein the gap g, the thickness t (mm) of each of the metal sheets, and the diameter D (mm) of the shoulders (5, 9) of the rotating tools (1, 8) satisfy 0.1 − 0.09 × exp − 0.011 × D / t 2 × t ≤ g ≤ 1 − 0.09 × exp − 0.011 × D / t 2 × t, wherein the pair of rotating tools (1, 8) are rotated in opposite directions (Q, R) to perform friction stir welding, and wherein the numbers of revolutions S (rpm) of the pair of rotating tools (1, 8) rotated in the opposite directions (Q, R) are the same, and the ratio T/S of a welding speed T (m/min) of the rotating tools (1, 8) to the number of revolutions S of the rotating tools (1, 8), the gap G (mm) between the shoulders (5, 9), the diameter D (mm) of the shoulders (5, 9), and the thickness t (mm) of each of the metal sheets satisfy T/S ≤ 1 / 1000 × D/t × 34.5 − 32.2 × G/t / 53 − 3.4 × D/t.