To provide a tightening device with a loosening prevention mechanism capable of dispensing with a welding machine and preventing the loosening of a rotary tightening device easily and quickly.

This tightening device T with the loosening prevention mechanism is provided with a bolt 11, an engaging member 12 mounted to the bolt 11 so as to prevent relative rotation, a slip-off prevention member 13 for preventing the slip-off of the engaging member 12, and a rotation prevention member 14 for preventing the rotation of at least either of the engaging member 12 and the slip-off prevention member 13. The rotation prevention member 14 has a rotation regulation part 12b positioned on an inner side of a rotation locus of at least either of the engaging member 12 and the slip-off prevention member 13. The slip-off prevention member 13 has a washer 13a tightened to the rotation prevention member 14 with the bolt 11 and a rising part 13b erected on the washer part 13a. The slip-off of the engaging member 12 from the bolt 11 is prevented by locking the engaging member 12 by the rising part 13b.