A pretensioner for removing slack from a seat belt having a slack removing mechanism drive actuated in response to a detection signal indicative of a prescribed degree of deceleration of a vehicle, and pawl gears which, owing to a driving force produced by the drive, and meshed with a gear that co-rotates with a seat belt take-up reel, rotates the take-up reel in a direction that removes slack from the seat belt. When deceleration of a predetermined magnitude is sustained, as at the time of a collision, slack in the seat belt is removed reliably and the amount of tightening of the seat belt is substantially constant. As a result, the seat belt functions at maximum effectiveness to effectively restrain the passenger wearing the seat belt.