A retractor for use with a seat belt webbing, comprising: a retractor base (1); a substantially cylindrical-shaped winding shaft assembly (9) which is rotatably supported on the retractor base (1) and around which a webbing can be wound; a torsion bar (2) to be inserted through a center hole formed in the winding shaft assembly (9) in such a manner that one end thereof is connected to the winding shaft assembly (9) in a mutually unrotatable manner and the other end thereof is connected to a locking member (15) in a mutually unrotatable manner; an emergency lock mechanism (300) which, in a vehicle emergency, connects the locking member to the retractor base to stop the rotation of the torsion bar to thereby prevent the winding shaft assembly from rotating in the webbing pull-out direction thereof, the emergency lock mechanism (300) being structured such that, when the emergency lock mechanism (300) is in operation and also a tensile force acting on the webbing exceeds a given value, the emergency lock mechanism (300) can absorb a shock produced in the body of an occupant by means of the torsional deformation of the torsion bar (2); a slit (3a), (30a) which is formed in the winding shaft assembly and extends in a direction crossing the winding shaft assembly while bypassing the insertion position of the torsion bar; and a large diameter portion (210) being larger in size than a width of the slit and including an loop-shaped end portion of the webbing.