For the driver and passenger seat belts, the structure inside the belt buckle covers is improper, and therefore, the posts inside the covers (to suppress noise while the seat belts are worn) can break from repeated bending of the section between the buckles and the cable guides, and a broken piece may be stuck in the lock mechanism. If this occurs, the lock mechanism may fail to work properly, and in which case, it could become impossible to engage the lock, or minor load such as a slight movement of the occupant could release the lock.