A friction stir welding method comprising: a first friction stir welding process of forming a first stirring region in a welded part of a metallic material, by moving a first rotary tool (21;101) in a first advancing direction while rotating the first rotary tool (21;101), the first rotary tool (21;101) being disposed on one side of the welded part in a thickness direction across the welded part; and a second friction stir welding process of forming a second stirring region in the welded part, by moving a second rotary tool (22;101) in a second advancing direction while rotating the second rotary tool (22;101), simultaneously with the first friction stir welding process, the second rotary tool (22;101) being disposed on the other side in the thickness direction across the welded part, wherein the first stirring region is a region which reaches an interior of the welded part from the one side of the welded part in the thickness direction, the second stirring region is a region which reaches the interior of the welded part from the other side of the welded part in the thickness direction, and the first stirring region and the second stirring region overlap each other inside the welded part in the thickness direction, characterised in that, when performing the second friction stir welding process simultaneously with the first friction stir welding process, a first rotary axis of the first rotary tool (21;101) and a second rotary axis of the second rotary tool (22;101) are provided in parallel, and the first rotary tool (21;101) and the second rotary tool (22;101) are disposed to be shifted to left and right in a width direction orthogonal to the first advancing direction and the second advancing direction, in an orthogonal plane orthogonal to the thickness direction. a first friction stir welding process of forming a first stirring region in a welded part of a metallic material, by moving a first rotary tool (21;101) in a first advancing direction while rotating the first rotary tool (21;101), the first rotary tool (21;101) being disposed on one side of the welded part in a thickness direction across the welded part; and a second friction stir welding process of forming a second stirring region in the welded part, by moving a second rotary tool (22;101) in a second advancing direction while rotating the second rotary tool (22;101), simultaneously with the first friction stir welding process, the second rotary tool (22;101) being disposed on the other side in the thickness direction across the welded part, wherein the first stirring region is a region which reaches an interior of the welded part from the one side of the welded part in the thickness direction, the second stirring region is a region which reaches the interior of the welded part from the other side of the welded part in the thickness direction, and the first stirring region and the second stirring region overlap each other inside the welded part in the thickness direction, characterised in that, when performing the second friction stir welding process simultaneously with the first friction stir welding process, a first rotary axis of the first rotary tool (21;101) and a second rotary axis of the second rotary tool (22;101) are provided in parallel, and the first rotary tool (21;101) and the second rotary tool (22;101) are disposed to be shifted to left and right in a width direction orthogonal to the first advancing direction and the second advancing direction, in an orthogonal plane orthogonal to the thickness direction.