Abstract: An in-vehicle communication system, comprises: a first in-vehicle communication device and a second in-vehicle communication device transmitting and receiving a signal to and from each other by a predetermined communication protocol related to Ethernet® (registered trademark), wherein the first in-vehicle communication device includes a PHY unit having a communication circuit, and a determination processing unit for determining whether or not the second in-vehicle communication device is in a link-down sleep state, when determined that the second in-vehicle communication device is in the sleep state, the PHY unit outputs a predetermined signal to the second in-vehicle communication device, and the second in-vehicle communication device includes a PHY unit having a communication circuit, a detection circuit for detecting the predetermined signal input to the PHY unit, and a power supply circuit for waking up the PHY unit when the detection circuit detects the predetermined signal.