A vehicle seat belt system includes a reversible belt tensioner (12) adapted to apply a belt tightening force (F) on several predetermined levels to a belt strap (14) of a seat belt, a control unit (16) for controlling the belt tensioner (12) and a device (18) for detecting the course of a road (S1, S2) ahead of the vehicle and for identifying bends lying ahead of the vehicle. The control unit (16) causes the belt tightening force (F) to be kept on an increased level when in the course of the road a further bend follows a just taken bend within a predetermined distance.