A friction stir welding device includes a pair of workpiece surface plates between which a gap extending along a welding line between workpieces is formed; a welding device body including a rotatable friction stir welding tool protruding upward from the gap; and a linear guide mechanism and a moving device for the welding device body. The moving device has a configuration in which a pair of pin racks extending in a direction along the gap are disposed at symmetrical positions on both sides in a width direction with a position immediately below a tool movement path of the friction stir welding tool as a symmetrical axis, and pin gears that individually mesh therewith are provided in the welding device body so as to be rotatably driven. Respective sets of a pin rack and a pin gear are alternately brought into a strong meshing state during the movement of the welding device body by causing the locations of pins and teeth thereof to deviate from each other by a half pitch.