A seat belt retractor includes a spool for winding a seat belt, a spring device for always biasing the spool in the belt winding direction, and a locking device which allows rotation of the spool when it is not activated and locks the spool from rotating in the seat belt withdrawing direction when it is activated. The spring device has a first spring always acting on the spool to bias the spool in the seat belt winding direction, and a second spring capable of acting on the spool to bias the spool in the seat belt winding direction. The seat belt retractor has a clutch for switching a mode of the second spring between an effective mode and an ineffective mode. The seat belt retractor further includes a control device which changes the winding torque during the seat belt winding operation.