In some embodiments, an input/output (I/O) circuit sends and receives a high-level signal and a low-level signal via a coupling capacitance provided on a communication line. The I/O circuit includes a receiving portion including a first detection circuit arranged to detect one of the signals and a second detection circuit arranged to detect the other signal, a transmitting portion including a three-value output circuit configured to output one of signals consisting of a high-level signal, a low-level signal, and a high impedance signal, and a control circuit configured to control the receiving portion and the transmitting portion. The control circuit judges a level of an inputted signal depending on detection results of the first detection circuit and the second detection circuit in a receiving state and controls an output value of the three-value output circuit in a transmitting state.