An in-vehicle communication system includes first and second switch devices that perform a relay process of relaying data in an in-vehicle network, the first switch device includes a switch unit and a processing unit that performs the relay process via the switch unit, the switch unit transmits target data to the second switch device instead of outputting the target data to the processing unit when an abnormality in the processing unit is detected, the target data being data which is received from a device other than the second switch device and is to undergo the relay process performed by the processing unit, and the second switch device performs a proxy process of relaying the target data received from the switch unit instead of the processing unit.