A seat belt device (1) including an obstacle detection portion for detecting an obstacle in an area surrounding a vehicle, a collision prediction portion (21) for predicting a probability of a collision between the vehicle and the obstacle based on a detection signal from the obstacle detection portion, a pretensioner device (40), and a pretensioner control portion (22) which controls, when a collision prediction is received from the collision prediction portion (21), the operation of the pretensioner device (40) such that seat belt is retracted prior to the collision. Multilevel tensions for retraction of the seat belt are set for the pretensioner device (40), and furthermore, the tension of the seat belt can be change by the pretensioner control portion (22) in accordance with a collision prediction level determined by the collision prediction portion (21).