It is possible to update firmware of domain masters during travelling. An in-vehicle communication system includes a plurality of domain masters, and a redundant domain master configured to be able to perform alternative operations of the plurality of domain masters. The domain masters transmits operation information to the redundant domain master prior to update of firmware. The redundant domain master executes the alternative operation of the domain master using the received operation information. The domain master receives, after the update of the firmware, operation information generated in the alternative operation from the redundant domain master and operates in the updated firmware.