It is an object to provide a retractor for a seat belt having a locking means and a shaft which can be twisted and exhibiting satisfactory safety. A retractor 1 for a seat belt according to the present invention having a locking means 100, which is capable of inhibiting rotation of a bobbin 3 around which a webbing 50 is wound in a direction in which the webbing 50 is drawn out in an emergency of a vehicle, a torsion bar 200A is disposed between the bobbin 3 and the locking means 100 and arranged to be twisted to respond to the instance wherein a predetermined load is applied to the locking means 100 so as to rotate the bobbin 3 in the direction in which the webbing 50 is drawn out. The retractor 1 for a seat belt has a rotation restraining mechanism 300 for restraining rotation of the bobbin 3 when the bobbin 3 has been rotated by a predetermined angle in the direction in which the webbing 50 is drawn out to correspond to torsion of the torsion bar 200A.