To improve looseness-preventing effect of a fastener.

In this fastener, a threaded hole 12 of a nut 1 having an action face in the periphery engaged with a tool is formed eccentrically to an central axis of the nut 1. Both end faces 6, 7 of the nut 1 are formed with circular recessed face parts 8, 9 whose peripheral end parts 8a, 9a are inscribed in peripheral end parts of the end faces 6, 7. Opening end parts 12b, 12c of the threaded hole 12 are located in the recessed face parts 8, 9. When fastening the nut 1 and the bolt 2 with the recessed part face 9 faced to a fastened member 3, the threaded hole 12 of the nut 1 is nearly elliptically deformed, an the bolt 2 is pinched in the threaded hole 12 of the nut 1 to closely contact a female part of the nut 1 with a male part 2a of the bolt 2.