A locking ring (24) is attached to the outer periphery of a union nut (23) so as to be incapable of relative rotation and relative axial-direction movement； peaked locking parts (24g), formed by obtuse-angle intersections between first inclined surfaces (S1) and second inclined surfaces (S2) inclined in opposite directions, are provided at a constant pitch in a circumferential direction to the inner peripheral side of a cantilever supporting spring ring (24c) provided to the locking ring； one or more engaging protrusions (25), with which the locking parts (24g) engage, are provided to the outer periphery on the proximal-end side of an external thread (14) provided to a connecting counterpart member； the locking parts (24g) move beyond first cam surfaces (25a) of the engaging protrusions (25) when the union nut (23) rotates in a tightening direction； and when the union nut (23) rotates in a loosening direction, the locking parts (24g) come into contact with second cam surfaces (25b) of the engaging protrusions (25), preventing the union nut (23) from loosening. The union nut (23) can be replaced by a bottomed cylindrical cap (26).
