When the seat belt tensioner operates, it instantly retracts the seat belt, enhancing its ability to immediately restrain the occupant. Also, a selectable energy absorption device is fitted within the seat belt tensioner of the front passenger's seat, and this reduces the burden on the occupant's chest while properly maintaining the restraining force of the seat belt by adjusting the energy absorption characteristic to the optimum condition for the occupant using occupant weight information detected by a front passenger's weight sensor, ensuring the seat belt ability to protect the occupant.