The invention relates to a device for producing a weld seam on work pieces by means of friction stir welding, said device having a welding head (5) with a rotating friction stir welding tool (4), the welding head being guided along the joining region and supported with respect to the work piece (1). In said device an irrecoverable tension rod (3) is provided for support, said rod being able to penetrate the welding region, being supported with its lower end on the work piece (1) in a tensioned manner, and carrying a rail (3b) on its upper end for supporting a counter piece (6) that is mechanically connected to the welding head (5) or to the friction stir welding tool (4). An additional supplementary tool (9) working, when seen in the direction of welding, in front of the friction stir welding tool (4) is provided for removing the upper section (3e) of the tension rod protruding above the welding plane from the welding region.