To provide a fastening member with a looseness preventer having a simple structure, easy in handling and of not substantially completely returning a screw.

The fastening member at least includes a bolt 1 for arranging an outward expanding tapered hole 11 in an end part and having a plurality of grooves 12 communicating with the tapered hole 11, a box nut 2 for arranging a truncated cone-shaped projection part 21 in the inner bottom surface center for push-expanding the grooves 12 by being inserted into the tapered hole 11, and a capsule 3 containing an adhesive 4 inserted into and arranged in the tapered hole 11. The length of the plurality of grooves 12 is preferably changed for each groove. A ring-shaped sponge 5 is desirably installed by being inserted into a base part of the projection part 21 of the box nut 2, and a washer 6 formed with recesses and projections on the whole both surfaces is preferably installed together with the box but 2.