To provide a pretensioner with further improved safety of the device, wherein gas is appropriately allowed to escape in the event that the gas pressure within the pipe rises excessively. A pretensioner is attached to the outer side of the frame of a seatbelt retractor. A pressure container is connected to the base portion of a pipe of the pretensioner. A gas generator is stored within this pressure container. Multiple balls are provided within the pipe. A hole is opened in the side wall of the pipe at the portion positioned between the gas generator and ball. A rivet or other device is driven into this hole. The rivet breaks in the event that internal pressure exceeding a certain pressure (i.e., abnormal pressure) is applied to the pipe, and falls loose from the hole. Alternative embodiments of gas escape mechanisms are provided.