A method for enabling a presetter to connect connecting portions of belts having different widths, and for forming an endless connection of a flat belt with no defects such as the bend of the connecting portions. This method includes a first step of setting so that the connecting portions at both ends of the flat belt are placed on a lower mold surface of the presetter, and that, with the flat belt held from both sides by a wall part standing from the lower mold surface and a flat belt member having substantially the same compression property as the flat belt, an upper mold of the presetter is placed on the flat belt and the flat belt member, and then fixing the flat belt and the flat belt member by the presetter; and a second step of connecting the connecting portions at the both ends by pressing the presetter with the flat belt and the flat belt member fixed.