the upper end of the seat back 102 by a fastening means and has a hole 224 therein a guide shaft 226 which guides the belt 210 210 a is provided in the hole 224 of the casing 222 due to the belt guide members 220 the belts 210 and 210 a which are extracted from the retractor 110 can always move based on the constant positions on the seat back furthermore a fastening unit 310 is provided on the belts 210 and 210 a and fastens the belts 210 and 210 a to each other to support a user who is sitting on the seat the fastening unit 310 includes a tongue plate 320 which is provided on one for example 210 of the belts 210 and 210 a and a buckle 330 which is provided on the other belt 210 a the tongue plate 320 is removably fastened into the buckle 330 one end of the tongue plate 320 has a hook 322 which is removably locked to the buckle 330 a first insert slot 324 into which the belt 210 is inserted is formed in the tongue plate 320 at a side opposite to the hook 322 a first twist prevention slot 326 into which the belt 210 is also inserted is formed in the tongue plate 320 at a position adjacent to the first insert slot 324 and is slanted at an angle with respect to the first insert slot 324 to prevent the belt 210 from being twisted the belt 210 is successively inserted into the first insert slot 324 and the first twist prevention slot 326 thus the user can easily adjust the position at which the tongue plate 320 is disposed in addition when the belt 210 is not in use the.