The disclosed seat belt retractor may include a spool for retracting a seat belt, a torsion bar, a pre-tensioner, and a locking device. The torsion bar may disposed inside the spool, wherein one side of the torsion bar is connected to the spool, and configured to absorb energy of movement of an occupant while being deformed under torsion. The pre-tensioner may be positioned at one side of the spool and configured to generate a rotating drive force for rotating the spool in a retracting direction in a condition of rapid deceleration of a motor vehicle. The locking device may be capable of locking another side of the spool such that a rotation of the spool in a withdrawal direction is suppressed at both an operating time of the pre-tensioner and a non-operating time of the pre-tensioner.