PURPOSE: To ensure tight sealing ability by interposing a union provided with a hard metallic sleeve between a coupling main body and a fastening nut and providing a lance for temporarily fastening a pipe on the outer peripheral portion of the sleeve.

CONSTITUTION: A large diameter opening portion of a unit 8 formed so that a ring- shaped edge 5 is provided on the bottom of the inner peripheral surface of the large diameter opening portion, a conical inner surface 6 is provided on the inner peripheral end of a small-diameter opening portion, and a hard metallic sleeve 19 having a lance 24 for temporarily fastening a pipe is disposed at a fixed gap from the small-diameter portion on the inner surface of the central portion is screwed in the conical end surface 1 of a coupling main body 3. The conical end surface 1 and the edge 5 are brought into press-contact with each other to provide sealing effect, a soft metallic ring 18 is brought in press-contact with the conical inner surface 6, a fastening nut 14 having a conical inner surface 13 is screwed in, and the ring 18 is reduced in its diameter to clamp a pipe 30 inserted between the ring 18 and the sleever 19 and fix the same to provide sealing effect. In the case of rotating the pipe 30, plural slits are provided on one end surface of the ring 18 to facilitate biting into the pipe 30, so that the fastening force can be increased.