A pre-tensioning device for a seat belt with a piston which is guided in a bent tube, and which in case of triggering is influenced by gas generated by a gas generator located in one section of the tube, to which a cable led into the tube by a cable opening is attached. The gas generator section (12) of tube (10) which accommodates gas generator (16) is offset from the piston path section (11) of tube (10) which accommodates piston (17) by creation of a narrowed cross-section inside the tube with a defined gas flow path (29, 41).