Plural electronic control units (ECUs) are connected to each other by an electric power line that is for supplying electric power. Each ECU sends information with a carrier wave having a first frequency lower than a dip frequency estimated by the physical condition and the circuit structure of electric power line, or having a second frequency lower than the first frequency. Or, the physical condition and the circuit structure of electric power line are determined, on the contrary, to make the dip frequency become higher than the frequency of utilized carrier wave. The frequency utilized for sending information may be selected in accordance with the contents of information.