A pretensioning device (10) for a safety belt, of a motor vehicle, having a gas generator (17), a piston (21) guided in a tube (16), which closes a pressure area (20) in the tube (16) that is pressurized by the gas generator (17) whereupon the piston (21) is able to move in a pretensioning movement which is able to be conveyed to the safety belt by means of an inertia body (19a) resting on a front face (9) of the piston (21). The piston (21) forms a pass-through opening (1) releasable upon exceeding of a predetermined pressure and a recess (26) is provided in the front face (9) with an inertia body (19a) resting thereon, so that a flow connection is created from the pass-through opening (1) to the area (25) located behind the piston (21) in the direction of the pretensioning movement (S).