Abstract: A nut and a bolt are provided, which make it difficult to be loosened once they have been tightened. A security nut includes: a nut body having an inside screw; and a plurality of torque transmission projections provided on an outer surface of the nut body along a circumferential direction. Each torque transmission projection has a torque transmission surface provided on an upstream side in a tightening direction of the security nut, and a release surface provided on a downstream side in the tightening direction of the security nut. A ridge line is formed between the release surface and the torque transmission surface, the ridge line inclining toward the upstream side of the tightening direction along the circumferential direction, from an advancing-direction end of the nut body to a retracting-direction end of the nut body.

Abstract: A nut and a bolt are provided, which make it difficult to be loosened once they have been tightened. A security nut includes: a nut body having an inside screw; and a plurality of torque transmission projections provided on an outer surface of the nut body along a circumferential direction. Each torque transmission projection has a torque transmission surface provided on an upstream side in a tightening direction of the security nut, and a release surface provided on a downstream side in the tightening direction of the security nut. A ridge line is formed between the release surface and the torque transmission surface, the ridge line inclining toward the upstream side of the tightening direction along the circumferential direction, from an advancing-direction end of the nut body to a retracting-direction end of the nut body.