To provide a one-step coupling having a simple structure and eliminating possibility of generation of looseness.

An insert hole 12 for a pipe 54 is arranged in a coupling body 1 to be connected to a flow passage hole 10 communicating with an inside of a pipe 54. An O ring 22 is attached in a housing hole 20 connected with the insertion hole 12. A ring member 28 for holding the O ring 22 by abutting on a step portion 24 is inserted in a caulking hole 26 connected to the housing hole 20 through the step portion 24. A regulating ring 30 with a plurality of extremities of elastically deformable regulating tongues 32 projecting inwardly from an outer periphery of a pipe 14 is inserted into the caulking hole 26. A holding member 34 in the ring shape is inserted into the caulking hole 26 to inwardly caulk the coupling body 1 at an extremity side of the caulking hole 26, thereby fixing a holding member 34 to the coupling body 1.