A pretensioner for restraining an occupant with a seat belt by withdrawing a seat belt buckle upon occurrence of an impact on a vehicle. The pretensioner includes a wire. One end portion of the wire is connected to the seat belt buckle and the other end portion of the wire is connected to a piston. The pretensioner further includes a cylinder slidably accommodating the piston and a gas generator supplying gas into the cylinder. The pretentioner further includes a guiding member, having a bent passage portion, that bends the wire toward the direction of the cylinder. On an inner wall surface of the bent passage portion of the guiding member, convex portions are formed in a direction orthogonal to the sliding direction of the wire. The convex portions prevent the wire from being pushed in along the direction of the cylinder.