<p>[PROBLEMS] An anti-loosening screw sufficiently biting into a female screw in tightening to increase the amount of engagement, increasing frictional force between itself and the female screw. The anti-loosening screw can achieve excellent anti-loosening effect for a long period. [MEANS FOR SOLVING PROBLEMS] An anti-loosening screw has a head section (1) and a shaft section (2). At least one thread (3) formed on the shaft section has a crushed thread section (4) formed by crushing a part of the at least one thread. The width of the thread at the crushed thread section is greater than the width at the same height of a thread not crushed and is suddenly narrowed after gradually widened in the rotation direction of the screw.</p>
