PURPOSE: To eliminate the unstability of connection losses by having plugs and an adapter fixed always in the same position by the constitution consisting in providing flanges opposing to the end faces of the adapter to the plugs, and providing a dowel to the end face of the adapter and one of the flanges and fitting holes to the other.

CONSTITUTION: At the connecting, first a plug 6b is inserted into an adapter 7 from its one side and a positioning dowel 11b and a fitting hole 13b are fitted. Next, a plug 6a is inserted into the adapter 7 from its other side and while the connection losses of the connector are being measured, the plug 6a is rotated to successively fit the fitting holes 13a onto the positioning dowel 11a. The fitting hole 13a of the smallest connection loss is selected by rotating the plug 6a one turn in this way, after which cap nuts 8a, 8b are locked and fixed respectively to the adapter 7.