A seat belt retractor for an automotive seat belt includes a housing having a spool arranged therein for winding the seat belt therearound. The seat belt passes between fixed and movable clamp portions arranged within the housing such that, under a condition where a collision sensor activates a pretensioner for tightening the seat belt in an emergency, the movable clamp is moved toward the fixed clamp for applying squeezing pressure to the seat belt to prevent extraction thereof from the retractor housing. The pretensioner may include a gas generator which pressurizes a cylinder so as to move a piston mounted within the cylinder very rapidly. The piston is operatively connected with the movable clamp and the spool via a wire for effecting substantially instantaneous clamping of the seat belt.