Abstract: A lock nut device having a nut setting member fitted loosely to an outer circumferential part of a threaded shaft; split nuts divided in two in axial direction and fitted to envelope the outside of the nut setting member and fitted to the threaded shaft, the slit nuts having inner threads and a locking groove into which the nut setting member, at an inner surface of the nut setting member, is fitted and having an outer tapered thread at an outer circumferential part; and a lock nut having an inner threaded tapered surface in threaded engagement with the outer tapered threaded surface of the split nuts. To fix the nut setting member the threaded shaft, the nut setting member is placed around the outer circumferential part of the screw shaft and tightened to a prescribed position. The split nuts divided in two are fitted so as to envelop the outer circumference of the screw shaft and the nut setting member.