Abstract: A belt tensioner, in particular a buckle tensioner, for a vehicle occupant restraint system having a belt, comprises a housing, a gas generator for generating compressed gas in the case of restraint, a working space into which the compressed gas flows, a piston part displaceable by the compressed gas and a belt engaging device. The belt engaging device is in force-transferring relationship with the fastened belt. The tensioner further comprises a traction transfer device extending through the working space, which traction transfer device connects the belt engaging device with the piston part, and a sheathing for the traction transfer device which at least partially covers the traction transfer device within the working space. The sheathing is of a material having a temperature stability with respect to the hot compressed gas.