To provide bolt and nut which perfectly improve tightening performance hardly loosen by hand once tightening, and never cause accidents due to loosening or falling of them, and also provide a tightening tool which does not cause a falling accident during working because it always fits to the bolt and nut during tightening work, and enables efficient tightening.

Bolt has axle portion serewing male thread on its outer surface and a head portion with polygonal upper face, provided at the top end of the axle portion, in which the tightened surface of the number corresponding to that of the side of polygon are provided on its outside face. The tightened surface of the head portion with polygonal upper face are deformed by a prescribed angle in the screwing rotation direction of the male thread to form a parallelogram surface.