In an I/O card has a case having metallic top and bottom covers, a printed circuit board mounted in the case, a connector disposed in the case for connecting with a personal computer system, and a card side I/O connector disposed in the case for mating with a cable side I/O connector connected to external device, the card side I/O connector is provided with no shell but is in direct contact with the metallic top and bottom covers, so that the both covers came in direct contact with a metallic shell of the cable side I/O connector when the cable side I/O connector is mated with the card side I/O connector. In an embodiment, the top cover is provided with locking pieces which engages with elastic locks of the cable side I/O connector for locking a mating condition between the card side I/O connector and the cable side I/O connector.