To provide an inexpensive anti-loosening structure having simple construction for achieving reliable anti-loosening effects.

In the anti-loosening structure, a combination of a screw 3 and a nut member 30 are fixed to each other with a work held between a washer 20 inserted through the screw 3 and the nut member 30. The washer 20 is formed of a resin and a string band material 23 to be put into a mounting hole 2 of the work is integrally formed thereon. Still, the band material 23 is long enough to be put into a female threaded hole 31 of the nut member 30. The band material 23 is held between the screw 3 and the female threaded hole 31 of the nut member 30 by screwing the screw 3 therein. Reliable anti-loosening effects are achieved only by threading the screw to the nut member. No need exists for doubling the nut member and arranging the anti-loosening member and the nut member in series, resulting in a reduced length. With the female threaded hole of the nut member put into the mounting hole, the nut member is less protruded and the screw is relatively more threaded thereto.