A seat belt retractor for a seat belt includes a spool for winding the seatbelt, a first seat belt load limiting mechanism for limiting load applied on the seat belt in an emergency, a second seat belt load limiting mechanism for limiting the load applied to the seat belt when a predetermined condition based on an emergency situation is satisfied in the emergency, a driving member for actuating the second seat belt load limiting mechanism, and a controller for controlling the driving member. When the predetermined condition is satisfied, the controller sets the second seat belt load limiting mechanism in an operative state by not actuating the driving member. When the predetermined condition is not satisfied, the controller sets the second seat belt load limiting mechanism in an inoperative state by actuating the driving member.