In a controller device, when a first transmitting signal is at a "L" level, a first operation voltage is high and the amplitude of signals CK and ICK is large, and on the contrary, the amplitude is small when the first transmitting signal is at a "H" level. In a data carrier device, the signals CK and ICK are subjected to full-wave rectification by a rectifier circuit so as to generate a second operation voltage, and a first receiving signal is extracted from the second operation voltage by a first signal detection circuit. On the other hand, in the data carrier device, when a second transmitting signal is at a "L" level, impedance between two contacts is small and the amplitude of the signals CK and ICK is small, and on the contrary, the amplitude is large when the second transmitting signal is at a "H" level. In the controller device, change of the amplitude of the signal ICK is extracted as a second receiving signal by a second signal detection circuit. Accordingly, supply of power and clock and two-way serial data communication can be simultaneously executed between the controller device and the data carrier device through merely two contacts.