A technology is provided which is useful to reliably warn the vehicle occupant by means of a seat belt in a seat belt retractor that is mounted in a vehicle and retracts / withdraws the seat belt for restraining a vehicle occupant depending on the driving of the electric motor. A seat belt retractor 1 of a seat belt device 100 to be mounted in a vehicle controls the electric motor so that the operations for increasing and decreasing the tension acting on the seat belt 3 by the spool are repeated several times at the time of fastening of the seat belt, to warn the vehicle occupant C, and the controller controls the electric motor so that a motion of the spool becomes relatively larger in a seat belt retracting direction in the first operation of the operations repeated several times (Fig. 1).