To provide an on-board communication system capable of easily preventing an existing electronic control device from making a malfunction, without adding a software change, when adding a power source supply state to respective parts in a vehicle from an on-board power source in response to a specification change in the vehicle.

This on-board communication system 10 includes a relay device 30a provided between an ECU 20a for determining whether or not to allow execution of a functional program of itself based on an existing operational power source condition and a vehicle inside LAN 5, for filtering a communication frame sent via the vehicle inside LAN 5 from another ECU 20c in response to a new operational power source condition. Thus, an erroneous determination of the ECU 20a caused by addition of a power source supply state (PG-IN), can be prevented only by adding the relay device 30a for performing simple processing, without adding a change to the existing operational power source condition in the ECU 20a.