A seat belt retractor is provided with a base having a pair of side plates, a bobbin coupled with a webbing, a torsion bar rotatably supported on the side plates of the base, one end of the torsion bar being coupled with the bobbin, and a locking mechanism coupled with another end of the torsion bar, the locking mechanism connecting the another end of the torsion bar with the base in a first state and not connecting in a second state. The torsion bar includes a torsionally deformable portion between two coupling portions thereof, and at least a part of the torsionally deformable portion is located outside beyond the side wall of the base.