A seat belt retractor is formed of a base frame, a reel shaft rotatably supported by the base frame, a locking member for locking rotation of the reel shaft to prevent the webbing from unwinding at a collision, and an energy absorbing member for absorbing impact energy against a vehicle occupant. The energy absorbing member allows a predetermined length of the webbing to be drawn out with a certain tensile load held on the webbing only when reaction forces acting on the locking member exceed a predetermined threshold.