PURPOSE: To facilitate tap processing and also to dispense with removing of chips after the processing, so as to reduce processing man-hours by forming cut grooves provided with cutting edges on a looseness preventing thread plug for a ring nut by remaining the rear end part from the tip part.

CONSTITUTION: A thread plug 30 is screwed into a threaded hole 28 after a ring nut 26 is so fastened that the constant pre-load may be applied from the outside of an angular ball bearing 18. The tap processing is applied to one part on the inner periphery of the threaded hole 28 and one part on the inner periphery of the ring nut 26 along the thread surface by cutting edges 33 of cut grooves 32 by screwing of the thread plug 30. Therefore, the thread plug 30 is screwed without applying forcible force to the ring nut 26 and fastened to the specified position. At this time, chips generated in the tap processing are accumulated in the cut grooves 32 and the prepared hole processing space part, and the chips are completely sealed in the threaded hole 28 at the time when the whole cut grooves 32 enter the threaded hole 28 by screwing of the thread plug 30.