A seat belt for constraining an occupant to a seat is provided with both a tension varying means 16 allowing for providing a tension to the seat belt to vary, and a tension detecting means for detecting an actual tension on the seat belt. There is provided failure detecting means, which detects failures of at least one of the tension varying means and the tension detecting means by comparing the tension provided by the tension varying means to the actual tension detected by the tension detecting means the difference in tensions thereof exceeding a predetermined value.