A pretensioner is disclosed which, when a vehicle suddenly decelerates, a large quantity of gas is generated, and the gas flows into a cylinder from a gas-generating a device. As a result of pressure of the gas, a movable pulley moves instantaneously together with a piston, so that the wire trained around the movable pulley is pulled. An amount by which a wire is pulled is twice an amount by which a wire is pulled in cases where the wire is directly connected to the piston, as in conventional devices. Accordingly, the wire can be pulled by the same amount by using only half of a length of a conventional cylinder, thus making it possible to manufacture a pretensioner which has a short cylinder.