To improve the pipeline connecting workability in the structure that a pair of metal tube bodies to be connected are connected to each other through a narrow lock ring of a connector by fastening a fastening nut by providing a recessed groove in one tube body to be connected with a constant space, and forming several pressure projections in an inclined surface of the fastening nut.

A connector 1 is formed with a main body screw 2 and a connecting screw 3 in both sides thereof, and a narrow lock ring 4 forming an inclined surface is formed in an outside end of the connecting screw 3. A fastening nut 5 is provided with a screw 6 to be threaded with the connecting screw 3 of the connector 1 and an inclined surface 7 to be bonded to the narrow lock rung 4, and a tube body A to be connected is connected to the barrow lock ring 4 by fastening the fastening nut 5. At this stage, a recessed groove 11 is provided in the periphery of the tube body to be connected with a constant space from an end surface. Further, several pressure projections 8 are projected in the inclined surface 7 of the fastening nut 5. With this structure, at the time of fastening the nut 5, the pressure projections 8 are engaged with the recessed groove 11 so as to display the loose-out preventing effect.