There is provided an input/output device which is capable of not exerting any adverse influence on other expansion devices connected to a system bus at the time of insertion or removal thereof. An expansion device 800 includes an electronic circuit 400 and a MOS switch 300 and is connected to a system bus (BUS) via a connector having long and short pins. The expansion device 800 is coupled to two power supply systems, namely a stable power supply 250 and an unstable power supply 260. At the time of insertion or removal of the expansion device 800, power is provided to the MOS switch 300 and a high impedance maintaining circuit from the stable power supply via a pair of long pins, so as to reliably place the MOS switch 300 in a high impedance state; and, inside the expansion device, the high impedance maintaining circuit 350 drives an open/close control terminal, and power is provided to the electronic circuit 400 from the unstable power supply 260. At the time of insertion or removal, adverse influence is not exerted on the signal transmission on the system bus, and effects of load variation on the main power supply are reduced.