A pre-tensioner capable of effectively absorbing kinetic energy exerted on a coupling member connected to a piston during operation. As a gas generator is actuated to generate gas in the event of emergency, the gas is introduced into a pressure chamber. By gas pressure of the gas, a piston moves at high speed toward a taper portion of a cylinder to reach the taper portion. According to the movement of the piston, a buckle is pulled toward a bracket so that the buckle is about to reach the bottom. After that, the piston further moves to the left while deforming the taper portion. The deformation of the taper portion absorbs kinetic energy of the piston. Therefore, kinetic energy of the buckle is absorbed so as to stop the buckle softly and the impact is buffered.