To provide a friction stir welding method which does not deteriorate the formability of an aluminum alloy sheet having a high r value for press forming.

The friction stir welding method of the aluminum alloy sheet uses a rotary joint tool 3 equipped with a circular column like base, a shoulder portion provided in a joined member side of the base, a probe provided so as to be vertically suspended in a coaxial direction with the base from the surface of the joined member of the shoulder portion. The aluminum alloy sheet is joined under the condition that the shape of the friction stir welding face of the probe is an approximately planar shape, the advance angle of the rotary joint tool 3 is 0° or more and 1.5° or less, the rotational speed of the rotary joint tool 3 is 700 rpm or more and 3,000 rpm or less, the joining speed is 300 mm/min or more and 1,800 mm/min or less, and the ratio of the rotational speed to the joining speed is 1.7 or more and 3.0 or less.