A case containing an optical interface section including a connector-fitting unit in its front portion, a printed board in its central portion, and an electrical interface section in its rear portion is accommodated in a housing. When a data link module is inserted in a cage, a housing tongue and a through hole both formed in a bottom plate of the housing are overlapped with a cage tongue and a lock pin both provided in and on a bottom plate of the cage, into and from which the data link module is inserted and removed, such that the lock pin is engaged in and locked by the lock hole. When removing the data link module from the cage, by angularly turning a bail pivotally supported by the connector-fitting unit, the cage tongue is pushed down following to the housing tongue with the camming operation of a plate cam, thus causing the lock hole to be disengaged from the lock pin.