The present invention provides an input/output device capable of bringing a per-unit input/output circuit into a simple configuration without impairing reliability even when logic levels opposite in polarity are outputted between input/output devices made conductive to the outside. The input/output device is equipped with one reference port Pk selected from a port group which inputs and outputs signals, target ports Pt selected from other than the reference port of the port group, and a conduction detector which detects that conduction is made between input/output terminals for the reference port Pk and the target ports Pt.