To provide a seat belt retractor equipped with a pretensioner mechanism compatible with an energy absorbing means arranged in a bobbin.

A seat belt retractor is provided with a cylinder 13 fixed to retractor bases 6a and 6b outside a bobbin 7, a helical shaft 11 having a helical groove 11a formed in its peripheral surface, a first transmitting member 9 fixed to one end of the helical shaft 11, a second transmitting member 8 for rotating the bobbin 7 in a direction for winding a waving in association with the movement of the first transmitting member 9, a piston member 15 including a hole having a projection meshed with the helical groove 11a of the helical shaft 11, a rotation regulating means 17 for regulating the piston member 15 to prevent its rotation, and a gas generator 19 for moving the piston member 15.