Abstract: Lap seat belt and shoulder seat belt are automatically set to restrain a seated person. A first end of each belt is slidable in a guide rail diagonally disposed on the inner surface of the door. Second ends of the belts are fixed on the inner side of the bottom seat and on the inner portion of the seat back respectively. When the first ends of the seat belts are in the highest position of the guide rail, the seated person is in a free state. When the first ends are slided and set in the lowest position, the seat belts restrain the person.