An input/output circuit has a first load having one end coupled to a first standard voltage line, a first MOS transistor having a drain electrode coupled to another end of the first load, a second load having one end coupled to the first standard voltage line, a second MOS transistor having a drain electrode coupled to another end of the second load, a third MOS transistor having a source electrode each of which is coupled to source electrodes of the first and second MOS transistors, a first constant-current source coupled between the source electrode of the first MOS transistor and a second standard voltage line, and a second constant-current source coupled between the source electrode of the second MOS transistor and the second standard voltage line. The circuit size is reduced by transmitting a differential signal or a single-ended signal using a single input/output circuit.