LOOSENESS PREVENTION SET

PROBLEM TO BE SOLVED: To provide a looseness prevention set which, with regard to the looseness prevention set for reliably preventing the looseness of a screw by mechanical engagement of a member without depending on a frictional force, solves such a contrary demand that the dismantling is also facilitated when disassembly is required, has high reliability applicable for both of bolt and nut and has high convenience on the handling.SOLUTION: A looseness prevention set by the use of a bolt or a nut includes: the bolt or the nut in which a plurality of radial direction grooves are disposed on a fastening surface; and a washer which has a plurality of radial direction protrusions having a shape engaged with the grooves and has spring property. The looseness prevention set is characterized in that a folding part is disposed on one of the protrusions of the washer, a recessed part with which the folding part is engaged is disposed on a member to be fastened and, therein, the number of the grooves disposed on the bolt or the nut is differentiated from the number of the protrusions in the radial direction disposed on the washer.