To provide a fastening device with a looseness preventing mechanism having a bent washer, for hardly causing the deformation of a fastened member even when the fastened member is formed of a thin plate or a soft material.

The fastening device T with the looseness preventing mechanism comprises a rotary fastener 11, the bent washer 12 for preventing the rotation of the rotary fastener 11, and a seat 13 having greater rigidity than the bent washer 12. The seat 13 is arranged on the reverse side of the bent washer 12 so that an abutting face 13b on the reverse side of the seat 13 abuts on the surface of the fastened member 2.