In a coupling mechanism, an inner peripheral wall of a coupling main body is inscribed with an outer side ring groove of containing an outer peripheral side edge portion of a stopper member, a stopper member introducing port formed by a width wider than a width dimension of a strip member of the stopper member is inscribed at a position of an outer wall portion of the coupling main body slightly shifted from a position of inscribing the outer side ring groove to a side of an inserting front end, the stopper member is applied with a spiral torsion such that both end portions opposed to each other of the strip member slightly carry out core deviation, an inner peripheral wall of the outer side ring groove is formed by a taper face in a state of being expanded from a rear end side to a front end side in an inserting direction, also an outer peripheral wall of the stopper member is formed by a taper face of a shape in correspondence with the taper face of the outer side ring groove, the inner peripheral wall of the stopper member is formed by a shape substantially in parallel with an outer peripheral wall of the connected pipe, and a face of the inner peripheral wall is formed by a rough face achieving a slip preventing effect.