Abstract: A reinforcing structure for a three-point type seat belt can be installed to a double seat in such a manner that a seat belt fixing bracket attached to the double seat is also united with a seat frame of a single seat disposed near the double seat, thereby securing a sufficient strength for the three-point type seat belt without the requirement of another reinforcing structure. In the reinforcing structure, the first second extension brackets are affixed to single and double seats respectively, while the first extension bracket is united with the second extension bracket.