A seat belt retractor with a torsion bar which can receive a large torque and can be easily formed by forging is provided. A reel assembly of the seat belt retractor includes a reel, a torsion bar, and a locking member. A first connection part of the torsion bar non-rotatably couples with concavities of a locking base of the locking member. A second connection part non-rotatably couples with concavities of the reel. The outer peripheries of the connection parts and are torx-shaped having a plurality of concavities and convexities formed in a wave-shape. A ratio R1/R2 of the distance between the tops of the wave-shaped convexities of the connection parts and the axial center of the torsion bar (radius R1) to the distance between the bottoms of the concavities and the axial center of the torsion bar (radius R2) is set to 1.2 to 1.3.