A pretensioner for introducing a webbing wound in a spool by explosive power obtained by exploding gunpowder when an impact is applied from the outside, the pretensioner includes: a hollow cylinder having one side surface for inserting a portion of a pinion connected to the spool; a micro gas generator (MGG) mounted in an upper end portion of the cylinder and for transferring explosive power into the cylinder while gunpowder provided at the inside explodes when an impact is applied from the outside; a rack mounted in a length direction at the inside of the cylinder and having one end portion disposed toward the MGG and having the other end portion disposed opposite to the pinion and having gear teeth provided in one side surface separated not to engage with the pinion before receiving explosive power from the MGG; and a roller for receiving explosive power from the MGG and rotatably mounted at the other side surface opposite to one side surface of the cylinder in which a portion of the pinion is inserted so that the gear teeth of the rack engage with the pinion when the rack falls toward a lower end portion of the cylinder and for guiding the separated rack to move toward the pinion.