Abstract: An object of the present invention is to provide an unfastening prevention device which has nothing projecting outside, and can prevent unfastening at either head or distal end of a bolt. An unfastening prevention is attained by covering a cap (6, 16) to screwed up fastening means formed of a bolt (2) and a nut (3). In a groove (6a, 16a) formed on an inner surface of the cap (6, 16) an outer periphery of a spring ring (8) is engaged, an inner periphery of the spring ring being engaged in a groove (7a) of a holder (7), restricting axial direction (2a) and resulting in slip out prevention. Even when a torque is given on the cap (6, 16), the cap idles, fastening condition does not change and unfastening prevention is attained.

Abstract: An object of the present invention is to provide an unfastening prevention device which has nothing projecting outside, and can prevent unfastening at either head or distal end of a bolt. An unfastening prevention is attained by covering a cap (6, 16) to screwed up fastening means formed of a bolt (2) and a nut (3). In a groove (6a, 16a) formed on an inner surface of the cap (6, 16) an outer periphery of a spring ring (8) is engaged, an inner periphery of the spring ring being engaged in a groove (7a) of a holder (7), restricting axial direction (2a) and resulting in slip out prevention. Even when a torque is given on the cap (6, 16), the cap idles, fastening condition does not change and unfastening prevention is attained.