To provide a coming-off stopper ring having excellent joining operability even in a narrow place, capable of joining corrugated flexible pipes with each other compactly and easily which does not easily come off from a pipe joint in the case of receiving shock and the like, a joining structure of the pipe joint using this and a mounting method of the coming-off stopper ring.

A stopper 7 and inclined stoppers 5a, 5b are provided in an outer circumferential part of a main body 3. The inclined stoppers 5a, 5b are arranged at mutually opposed positions in the main body 3. Inclined parts 6a, 6b respectively provided for the inclined stoppers 5a, 5b are directed in opposite directions from each other. The coming-off stopper ring 1 is rotatable in a female pipe joint while deforming along inclination of the inclined parts 6a, 6b and the inclined stoppers 5a, 5b can be fit to a coming-off stopper ring mounting part 16.