Linear pretensioning drive for a seat belt

A linear pretensioning drive for tightening a vehicle seat belt has a guide tube (1), a piston (2) which can be driven by the guide tube, and a reverse movement lock (3), formed by balls (8), which are arranged on a ring around the tube axis (4). In the drive direction in front of the reverse movement lock (3) at least one support part (5) is provided which lies against the inner wall (7) of the guide tube 1 in the locking position with in each case a greater supporting surface than each ball (8) of the reverse movement lock (3).