A belt tensioner (10) for a vehicle occupant restraint system comprises a tube (12) containing gas in the trigger case and an igniter unit (14) arranged in the tube (12) which igniter unit is inserted in an open end face (16) of the tube (12) and includes a base (18) and a cap (20) fastened to the base (18) for accommodating a propelling charge, wherein the cap (20) is arranged at least pertly inside the tube. The igniter unit (14) is welded to the tube (12).