To ensure sealing and firmness at a low cost and give excellent convenience by forming this device with a main body of a single block, and providing a packing box element cooperating with the outside casing of an outside coaxial cable at least on an outside connection end for receiving an outside cable end.

A connecting element includes a packing box unit constituted of a flexible gasket 34 on an outside connection end 11, a washer 35 and a cap 33 screwed to the main body 1 of the connecting element. When the gasket 34 is compressed to the inside of the cap 33, its end is strongly pressed to the end of the main body 1 of the connecting element, and its inside wall is strongly pressed to the casing portion of an outside coaxial cable inserted into a connection tail 27. The mechanical hooking the cable is reinforced, moisture is prevented from infiltrating into both faces of a connector and the cable via the connection end of the connecting element, and a completely sealed connection is made.