Provided is a pre-tensioner for a seat belt, the pre-tensioner including a housing, a carrier disposed on a first surface of the housing, a fixing member connected to the carrier, a first pulley disposed in a first zone of the first surface of the housing, a second pulley disposed in a second zone of the first surface opposite to the first zone with respect to the carrier, a driving unit having at least a portion coupled to the second pulley, and a belt disposed to mesh with the first pulley, the second pulley, and the carrier.