To cancel the possibility that airtightness inside rust prevention cap may be influenced by a temperature change.

A body section 21 covering the shank 5a of bolt 5 which is temporarily fixed to a bolt hole 3 of bottom steel plate 1 through a bolt temporarily fixing nut 14 is provided, moreover a thread section 23 screwed into the shank 5a of the bolt 5 is provided inside the body section 21 in a rust prevention cap 20 for bolt with an opening 22 attached in caulking to a cap attaching section 17 of the bolt temporarily fixing nut 14. Furthermore, the possibility that airtightness between the opening 22 and the cap attaching section 17 of the bolt temporarily fixing nut 14 may be influenced by an inner atmospheric pressure of the rust prevention cap 20 for bolt is canceled by compulsorily caulking the opening 22 to the cap attaching section 17 of the bolt temporarily fixing nut 14 by utilizing a tightening axial force with respect to the bolt 5 of the thread section 23 of the rust prevention cap 20 for bolt.