To surely prevent the looseness by singly using a bolt and also apply the looseness prevention technology to looseness prevention of a nut.

An insertion hole having a diameter equivalent to approximately 0.3 times of 'nominal size' is bored in a center axis of a male screw tip of a conventional bolt in a depth with a dimension equivalent to approximately 0.8 times thereof. 'A straight line groove' having a groove width size of 0.7-1.6 mm is processed in the same depth as the insertion hole. In the 'straight line groove' of the slot, 'an E-shaped snap ring' in JIS specification is angularly shaped. 'A specially manufactured E-shaped snap ring' of a shape containing a 'crest bending' or 'semicircular bending' part mutually fitted is inserted in and suitable for the insertion hole in a back part side surface on an opposite side to an opening in the snap ring. When the bolt is fastened to the mating female screw, the outer edge of the bolt pressed onto the inner diameter of a female screw to be locked by preload in an axially orthogonal direction through elastic expanding force of bending of the 'specially manufactured E-shaped snap ring' , wherein the rotation of the bolt is prevented through the medium of the slot at the tip of the male screw to prevent the looseness of the bolt.