To provide a bolt and a nut capable of preventing the bolt from being rotated following tightening of the nut without causing the complication of production.

The seat face 14 of the head part 11 of the bolt 10 is formed in such a shape that its outer side is axially thicker than its center side, and a frictional force between the seat face of the head part and a tightened member 30 is made larger than a frictional force between the seat face 21 of the nut 20 and a tightened member 31. Also, the seat face of the nut is formed in such a shape that its outer side is axially thinner than its center side, and the frictional force between the seat face and the tightened member is made smaller than the frictional force between the seat face of the head and the tightened member.