Abstract: A device for activating a seat belt pretensioner comprising a piston that can be driven by a propellant gas in a tubular piston guide. A pressure chamber, in which the propellant gas acts upon the piston and a weak point provided in an opening in a wall of the piston, which ruptures under excess working pressure in the pressure chamber. The weak point is a plastic deformable membrane, which is deformed into a dome shape under excess pressure in the pressure chamber and ruptures in the region of the dome-shaped deformation. The seat belt pretensioner has a belt reel associated with blocking teeth, and a blocking pawl. The time required for the membrane to be deformed and rupture is sufficient for the blocking pawl to securely engage the blocking teeth thereby blocking the belt reel against rotation in a direction of belt extraction.