Described is a belt tensioner for a seat belt having a belt retractor, a tensioner pinion (4) non-rotatably connected to the belt retractor, a tube (2) having a pyrotechnical gas generator arranged therein, and a plurality of mass bodies (6) driven by the pressure generated by the gas generator. The mass bodies (6) engage with the tensioner pinion (4) in a force transmitting manner and drive the belt retractor in the winding direction. A first and a second section (2a, 2b) of the tube (2) run parallel to each other. A third section (2c) with a curved course that projects out of the plane (E) connects the first and second sections, thereby creating a space (7) between the first, second and third sections (2a, 2b, 2c), for accommodating at least one of the mass bodies (6a, 6b).