A seat belt tightener has an electrical tightening drive that can be operated in dependence on sensor signals of sensors assessed in an evaluating device. The sensors detect specific vehicle dynamic conditions and transmit electrical indicating signals that are proportional to the detected condition to the evaluating device. The evaluation device compares the signals with allocated threshold values to determine whether or not a potential crash situation is present. Should a potential crash situation be present, the electrical tightening drive is controlled accordingly.