In the seat belt apparatus of the invention, a belt retracting motor is operated so as to retract the seat belt by a predetermined amount when it is determined that the vehicle will roll over based on a lateral acceleration, a roll rate and a roll angle which are detected and calculated by a lateral acceleration sensor and a roll rate sensor, according to a predetermined first determination condition. Also, a pretensioner is operated so as to retract the seat belt by a predetermined amount when it is determined that the vehicle will roll over according to a second determination condition indicating that there is a higher possibility of rollover. When it is determined that the vehicle will not roll over, operation of the belt retracting motor is cancelled, and the seat belt is permitted to be pulled out.