To provide a tightening device capable of easily obtaining a secure tightening condition even if a space on a back side of a tightened member is narrow and avoiding forgetting of tightening.

This tightening device 1 is provided with a master bolt 2, a slave bolt 3, a master nut 4, and a slave nut 5. A shaft part 30 of the slave bolt 3 is inserted into a slave bolt insertion hole 22 of the master bolt 2, and a tightening piece 31 protrudes in the direction out of diameter from a slit 24 to assemble the slave bolt 3 on the master bolt 2, and immediately after that, the slave nut 5 is screw-fitted in a tip protruding part of the slave bolt 3. After inserting the master bolt 2 into a bolt insertion hole 11 from the back side of the tightened member 10 in this condition, the master nut 4 is screw- fitted in the master bolt 2, and the master nut 4 and the slave nut 5 are retightened to nip and tighten the tightened member 10 from its surface and rear surface by the tightening piece 31 of the master nut 4 and the slave bolt 3.