To allow a seat belt retractor to be easily assembled and to be made compact while effectively operating a pretensioner and an energy absorbing mechanism. A plurality of balls 20 rotate a ring gear 25 by gas generated in the event of an emergency. Since internal teeth 25a are meshed with external teeth 26a, a pinion 26 is rotated by the rotation of the ring gear 25, and a spool retracts a seat belt. After pressing the ring gear 25, the balls 20 separate from the ring gear 25 and are stored in an arc-shaped passage 33. Since the seat belt is then withdrawn, the pinion 26 and the ring gear 25 rotate in a seat-belt withdrawing direction. In this case, some of the balls 20 in contact with the ring gear 25 are returned by the ring gear 25. Therefore, the ring gear 25 more smoothly rotates because the influence of the balls 20 is suppressed.