Provided is an anti-loosening spring nut including: a female thread spring which is wound a certain number of times along male threads of a bolt, and at whose lower portion and upper portion are projectingly formed, respectively, a lower catching loop end and an upper catching loop end, at directions different from each other and outward in a radial direction, and an external casing which has a shape of an enclosure accommodating the female thread spring, on a bottom plate of which is penetratingly formed a bolt insertion hole into which the bolt is inserted, and along whose side face are provided a plurality of pushing projections which are depressedly formed inward at constant intervals.