A seat belt retractor to be installed in a seat belt system of a vehicle for restraining an occupant on his seat by a seat belt in an emergency, comprises a reel, a torsion bar, one end of which is connected to the reel so as to be rotated therewith, the other end of which is formed into a locking portion, a lock member to be engaged with the locking portion for locking the other end of the torsion bar in an emergency and a stopper means which is disposed between the reel and the torsion bar for stopping the rotation of the reel. In an emergency such as upon an impact of the vehicle, the lock member operates to lock the other end of the torsion bar. The torsion bar of which one end is connected to the rotating reel is twisted to absorb the impact energy of an occupant. And before the torsion bar is broken, the stopper means operates to stop the rotation of the reel. Therefore, even if a large tension force is applied to the seat belt, the torsion bar is not broken due to the twisting thereof.