A pretensioner (1) for a seat belt system (7) of an automobile includes a gas generator (2), a gas pipe (3), a piston (5) and a coupling mechanism (6). The gas generator is adapted to generate a high-pressure gas when a shock occurs in the automobile due to a collision, a sudden stop, etc. The gas pipe is formed to receive therein the gas generator. The gas pipe is adapted to guide the high-pressure gas released from the gas generator to the piston. The piston is adapted to be moved or displaced by the pressure of the high-pressure gas guided by the gas pipe. The coupling mechanism is connected to each of the piston and a buckle (8) of the seat belt system (7). The coupling mechanism is adapted to wind up or pull a seat belt (9) in accordance with the movement of the piston to increase a restraining force of the seat belt.