To provide a nut with a tightening torque managing function that can increase workability by preventing surely the nut from loosening, and can easily check the completion of a tightening work by finding a given tightening torque in one process.

The gradient on the outside taper face 6 of a pipe part 4 and the inside taper face 7 of a tightening ring 2 are set in larger than 1 degree to smaller than 10 degrees. The outside taper 6 and the inside taper 7 are firmly pressed against each other to fit around about all surfaces so that the pipe 4 and the ring 2 can be temporally held each other. When the nut is not tightened, the end face 3 a of a head 3 is detached from the end face 2a of a tightening ring 2. When the nut is completely tightened, the end face 3a is pressed to the end face 2a each other by a given force.