There is provided a communication system that includes master and slave communication ECUs. The master communication ECU sends a clock signal to a clock communication line, with which the communication ECUs synchronize with each other when sending and receiving data. The master and slave communication ECUs then receive the clock signal through the clock line. The ECUs use the received clock signal as a reference timing, which is designated as a transmission/reception reference, and send or receive data to/from data communication lines.