To provide a friction stir welding device which is excellent in the efficiency of joining work, and at the same time, does not need a large-scale structure even when a joining member is formed widely.

The friction stir welding device 1 performs friction stir welding to inner corner portions 20a, 20b constituted by butting a horizontal member 22 arranged horizontally and a vertically arranged member 24 arranged vertically on the horizontal member 22, and further includes a pair of friction stir welding tools 10a, 10b arranged so as to face opposedly in the both sides sandwiching the vertically arranged member, first pressing mechanisms 18a, 18b which press the pair of the friction stir welding tools to the inner corner portions, and a moving mechanism which relatively moves the pair of the friction stir welding tools along butt lines 21a, 21b of the horizontal member and the vertically arranged member while leaving them facing opposedly, in the pair of the friction stir welding tools, while fixed shoulders 12a, 12b thereof are in a position facing opposedly sandwiching the vertically arranged member, stirring shafts 14a, 14b thereof are in a position shifted in the relatively moving direction of the friction stir welding tools.