A friction-stir welding tool includes a front shoulder; a rear shoulder in contact with plate materials; and a probe penetrating the plate materials between the front and rear shoulders, the probe connecting the front shoulder and the rear shoulder to each other. The probe has a first screw part provided on at least one axial end section, and at least one of the front shoulder and the rear shoulder has a second screw part in threaded engagement with the first screw part, the threaded engagement changing so that the rear shoulder and the front shoulder approach each other along the axial direction as the front shoulder and the rear shoulder rotate relative to each other, and the threaded engagement with the first screw part linking at least one of the rear shoulder and the front shoulder with the probe.