Abstract: A coming off preventing device is provided for a joining part of an air supply and exhaust pipe which joins two pipes by inserting an entry of one air supply and exhaust pipe into a socket of another air supply and exhaust pipe. A cut out window is provided at the socket side, and a metal stopper is equipped on the outer side of the cut out window, wherein the metal stopper has a lock unit which includes a push part on an upper surface of one end, a rectangular shaped lock part on a lower surface of another end and a bearing part on an upper surface of a center part. Further, the lock unit has a lower surface to which one end of a plate spring is welded wherein the plate spring is bent to have an obtuse angle. A joint cover covers said lock unit and fixes said lock unit axially. Further to the entry side, a circumferential groove is engraved to the opposite direction to the entry with a gradient, and an entry stopper.