A flexible detecting terminal 6 is provided within an inside connector 1 in the vicinity of the cylindrical outer conductor 5 which surrounds a central conductor 4. An outside connector 52 has a central conductor 52A for connection with the central conductor 4 and a conductive shell 52B fitted over the cylindrical outer conductor 5. A contact section 6B is provided on the detecting terminal 6 such that when the connectors 1 and 52 are not connected to each other, it is spaced from the cylindrical outer conductor 5 but when the connectors 1 and 52 are connected to each other, it is brought into resilient contact with the cylindrical outer conductor 5.