A seat belt retractor, which is capable of winding and unwinding a seat belt for occupant restraint by mechanism of an electric motor, removes slack of the seat belt and controls the restraining feeling on a vehicle occupant to be as little as possible. A seat belt retractor of a seat belt apparatus to be installed in a vehicle is adapted to control a motor to perform an action of increasing seat belt tension to wind up a seat belt which is excessively unwound from the spool when a vehicle occupant wears the seat belt and to be driven according to such a setting that, at the initial stage of the action of increasing the seat belt tension, the tension increasing rate until the seat belt tension reaches a target tension is lower than a predetermined rate.