Abstract: A bolt temporary fixing nut 9, one axial end of which has a smaller diameter than a bolt hole of a bottom steel plate 1 and an outer peripheral surface of which is made into a tapered shape which is increased in diameter from the one axial end toward the other axial end so as to have a larger diameter than the bolt hole 3, is formed. The bolt temporarily fixing nut 9 is threadedly attached to the bolt 5, which is inserted through the bolt hole 3 from the bottom, and the one axial end of the nut is forcibly screwed into a gap between the bolt hole 3 and the bolt 5, thereby temporarily fixing the bolt 5 to the bolt hole 3 so as not to rotate. Thereafter, after the splice plate 2 is placed on the bottom steel plate 1, the nut 6 is fastened to the bolt 5, thereby joining the bottom steel plate 1 and a splicing plate 2. According to the invention, it is possible to prevent the bolt, which is hold in the bolt held in advance, from co-rotating at the time of nut fastening.