A friction stir welding tool (100, 100a, 100b, 100c, 100d, 100e) configured to perform friction stir welding of first and second facing portions (w4, w5) each constituted by making a plurality of end portions of workpieces face each other, the friction stir welding tool (100, 100a, 100b, 100c, 100d, 100e) comprising: a stirring shaft; a first shoulder portion provided so as to be unrotatable relative to the stirring shaft and configured to rotate together with the stirring shaft when the stirring shaft rotates; a second shoulder portion provided so as to be unrotatable relative to the stirring shaft and configured to rotate together with the stirring shaft when the stirring shaft rotates; and a third shoulder portion attached around the stirring shaft so as to be located at a position between the first shoulder portion and the second shoulder portion, wherein: a first gap portion into which the first facing portion is inserted is formed between the first shoulder portion and the third shoulder portion; and a second gap portion into which the second facing portion is inserted is formed between the second shoulder portion and the third shoulder portion; characterized in that the third shoulder portion includes an inclination preventing portion configured to prevent inclination of the stirring shaft in a front-rear direction along a joining direction of the friction stir welding during the friction stir welding by contacting the workpieces to receive reaction force from the workpieces and in that the inclination preventing portion comprises at least one contact portion configured to contact the workpiece at a position outside first and second friction stir welding regions in a direction along a joining line of the first facing portion and a joining line of the second facing portion in the third shoulder portion, the first friction stir welding region being a region where the friction stir welding is performed at the first gap portion, the second friction stir welding region being a region where the friction stir welding is performed at the second gap portion. a stirring shaft; a first shoulder portion provided so as to be unrotatable relative to the stirring shaft and configured to rotate together with the stirring shaft when the stirring shaft rotates; a second shoulder portion provided so as to be unrotatable relative to the stirring shaft and configured to rotate together with the stirring shaft when the stirring shaft rotates; and a third shoulder portion attached around the stirring shaft so as to be located at a position between the first shoulder portion and the second shoulder portion, wherein: a first gap portion into which the first facing portion is inserted is formed between the first shoulder portion and the third shoulder portion; and a second gap portion into which the second facing portion is inserted is formed between the second shoulder portion and the third shoulder portion; characterized in that the third shoulder portion includes an inclination preventing portion configured to prevent inclination of the stirring shaft in a front-rear direction along a joining direction of the friction stir welding during the friction stir welding by contacting the workpieces to receive reaction force from the workpieces and in that the inclination preventing portion comprises at least one contact portion configured to contact the workpiece at a position outside first and second friction stir welding regions in a direction along a joining line of the first facing portion and a joining line of the second facing portion in the third shoulder portion, the first friction stir welding region being a region where the friction stir welding is performed at the first gap portion, the second friction stir welding region being a region where the friction stir welding is performed at the second gap portion. a first gap portion into which the first facing portion is inserted is formed between the first shoulder portion and the third shoulder portion; and a second gap portion into which the second facing portion is inserted is formed between the second shoulder portion and the third shoulder portion; characterized in that the third shoulder portion includes an inclination preventing portion configured to prevent inclination of the stirring shaft in a front-rear direction along a joining direction of the friction stir welding during the friction stir welding by contacting the workpieces to receive reaction force from the workpieces and in that the inclination preventing portion comprises at least one contact portion configured to contact the workpiece at a position outside first and second friction stir welding regions in a direction along a joining line of the first facing portion and a joining line of the second facing portion in the third shoulder portion, the first friction stir welding region being a region where the friction stir welding is performed at the first gap portion, the second friction stir welding region being a region where the friction stir welding is performed at the second gap portion.