An on-vehicle communication system, performing message authentication using transmitter and receiver codes that are message authentication codes respectively generated by a communication data transmitter and receiver, includes a first ECU, connected to an on-vehicle network, configured to have, among mutually different first and second encryption keys, only the first one and verify, by using the receiver code generated using the first one, the transmitter code assigned to the communication data received; a second ECU, connected to the on-vehicle network, configured to have at least the first one and transmit communication data to which the transmitter code generated using the first one is assigned; and a third ECU connected to the on-vehicle network and a network outside a vehicle, configured to have only the second one among the first and second ones and generate the transmitter or receiver codes using the second one when making communication over the on-vehicle network.