To provide a shield structure in which an electromagnetically shielded connection between a plug and a socket is formed when the plug and the socket are connected with each other, and a permanent and reliable electromagnetic shield can be realized by a physically simple method even if the connection is cut periodically.

An apparatus forms the electromagnetically shielded connection between a plug device 2 of a first unit (3) and a socket device 5 having a socket 4 of a second unit 6. A plug 1 of an adapter 3 as the first unit, and the socket 4 of a flight data recorder 6 as the second unit, form a plug connection part 7 when they are connected with each other. The plug connection part 7 is surrounded by a shield device 13 which exhibits good electromagnetic shield.