A belt tensioner for a seat belt system includes a belt shaft housing, a belt shaft pivoted about an axis in the belt shaft housing, a rope reel tightly connected to the belt shaft. A tensioner drive includes a tensioner tube, a deformable plastic element guided to be longitudinally movable in the tensioner tube, a drive pinion and a pull rope which is partly wound onto the rope reel. The plastic element is adapted to engage in the drive pinion and to drive the drive pinion in the tensioning direction. During rotation of the drive pinion the pull rope is wound onto a rope drum of the drive pinion, on the one hand, and is unwound from the rope reel, on the other hand, while the belt shaft rotates.