To improve the heat insulating effects of a double pipe structure by surely preventing air between an outer pipe and an inner pipe from flowing from the front end opening of the outer pipe at its connection portion to a joint to the outside, while preventing the entry of foreign matters such as dust and dirt from the front end opening of the outer pipe.

The end of a fluid pipe 11 is connected to an inside connection portion 5 of the joint 1, and the end of a clad pipe 21 is externally inserted and connected into an outside connection portion 6 formed on the outside of the inside connection portion 5 of the joint 1 to avoid a space S between the fluid pipe 11 and the clad pipe 21 from facing the outside. A come-off preventing means prevents the come-off of the clad pipe 21 connected to the outside connection portion 6. The come-off preventing means consists of an audit window 7 recessed in an outer face 6d of the outside connection portion 6 of the joint 1, and a U-shaped clip 31 fitted through the outside of the clad pipe 21 via its peripheral wall 22 to the audit window 7 for clamping the peripheral wall 22 of the clad pipe 21 together with the audit window 7.