A seat belt retractor includes a base frame, a spool journalled to the base frame, a webbing wound around the spool, and a motor for rotating the spool. A speed-reduction mechanism is interposed between the motor and the spool. The speed-reduction mechanism includes at least one transmission train with a predetermined gear ratio, a common transmission element connected to the transmission train, and a resistant torque device connected to the transmission train. The common transmission element transmits rotation of the motor to the spool when the torque of the rotation is smaller than the preset value. Winding of the webbing is made based on the wearing condition of the webbing and the running condition of the vehicle.