To provide a nut loosening preventive device for locking so that a screw is not loosen with workpiece being secured with a bolt and a nut.

The nuts (14) and (15) are locked so as not to be loosen by screwing the main nut (14) to a screw shaft part (12) of the bolt (11) inserted into a through-hole Q of the workpiece W to tighten it with a predetermined torque F1, next externally inserting a loosening preventive ring (18) to the screw shaft part (12), then screwing the auxiliary nut (15) and tightening with a predetermined torque F2. When the auxiliary nut (15) is tightened, a working face (19) reduces the diameter of the loosening preventive ring (18) to insert the inner periphery of the ring into the valley of a male screw thread (12a) of the bolt (11) and also form a clearance S between face surfaces (17a) and (17a) of both the nuts (14) and (15) via the ring, and in this condition, both the nuts (14) and (15) strongly interpose the loosening preventive ring (18).