A seat belt retractor includes a seat belt winding device for winding up a seat belt by actuation of a motor and a control unit that repeats a process of stopping the actuation of the motor on an overload of the motor and restarting actuation of the motor after lapse of a predetermined time period. The control unit includes a detection module for detecting an occupant getting on or off a vehicle and a determination module that, using criteria, determines whether or not actuation of the motor is to be restarted. The criteria when the occupant enters the vehicle is different from when the occupant exits the vehicle. The criteria are provided so that the determination module is more likely to determine that the actuation of the motor is not to be restarted when the occupant enters the vehicle as compared to when the occupant exits the vehicle.