A seat belt tensioner includes a guide, which has a pair of opposing plates connected by a connector, a wire, a first end of which extends upward to be connected to a seat belt, a central portion of which is curved to extend around the connector, and a retracting mechanism, which is connected to the second end of the wire and is operated by an external signal in an emergency. Here, the connector is integrally formed with at least one of the plates. Thus, the seat belt tensioner enables structural simplification, easy assembly, and a reduction in the number of parts.