A data transmitter receives a reference current from a current generator and outputs as a current signal a current obtained by multiplying the reference current by a given number in accordance with the value of transmit data. A data receiver, on the other hand, receives the current signal from the data transmitter to generate a receive signal, while receiving a reference current from the current generator to generate a reference signal which is necessary for level determination of the receive signal. In this manner, the reference currents, from which the current signal and the reference signal are respectively generated, are supplied from the current generator that is used in common by the data transmitter and the data receiver.