An electronic control unit and on-vehicle devices are each connected to a common power supply line via a communication module. The power supply line has positive and negative electrode side core wires. Each communication module is provided with a connection unit and a signal processor. The connection unit has a transformer having two pairs of input-output terminals, a first pair of which is respectively connected to the positive and negative electrode side core wires. The signal processor is connected to the second pair of input-output terminals of the transformer. The signal processor converts analog signals from the power supply line and outputted from the second pair of input-output terminals, converts externally inputted digital signals into analog signals and transmits the converted analog signals to the second pair of input-output terminals so that the converted analog signals are superimposed on the power supply line voltage.