A preloader in which one end of a wire, which is connected to a webbing take-up shaft via a clutch unit when a vehicle is abruptly decelerated, is fixed to a piston in a cylinder. The cylinder has a pair of through holes defined therethrough. The through holes are defined further toward a base of the cylinder than the piston is located, when the piston is held in abutment against a leading end of the cylinder. When the vehicle is abruptly decelerated, a large quantity of gas is generated by a gas generator and is introduced into the cylinder, thereby moving the piston to abut against the leading end of the cylinder. In this condition, the gas, which has been introduced into the cylinder, passes through the through holes so as to be discharged from the cylinder to an outside. Accordingly, the high pressure gas does not act on the piston.