A seat belt tensioner for motor vehicle, having a belt shaft (12) rotatably supported in housing as a carrier of a seat belt wound thereon and with a tensioner drive coupled thereto by a metal strip (18), the drive rotatable driving the belt shaft (12) when triggered by action on the metal strip (18) and the movement thereof, the metal band (18) before triggering the tensioning drive (10) surrounds an extension (13) of the belt shaft. The metal strip (18) is wound around the extension (13) of the belt shaft (12) in at least one winding turn serving as winding supply (20) for the movement thereof. The winding reserve (20) is held at a radial gap (21) to the extension (13) of the belt shaft (12) by a holding element (22) releasing the metal band (18) in case of tensile force acting on the metal strip caused by activation of the belt tensioner causing the metal stripto engage and rotate the belt shaft extension.