Provided is a vehicle-mounted relay device, a vehicle-mounted communication system, and a relay program configured to prevent an unauthorized message from being relayed between networks. A vehicle-mounted relay device includes a plurality of communication units to which a plurality of CAN buses are connected and determines whether a message transmitted by any ECU connected to the CAN buses are authorized. If a message is unauthorized, then the vehicle-mounted relay device 10 gives, to the respective CAN bus connected to the communication unit receiving the message, a notification that the unauthorized message has been transmitted. The vehicle-mounted relay device prohibits further relaying of a message that has the same CAN-ID as the CAN-ID contained in the message determined to be unauthorized. Concurrently, the vehicle-mounted relay device gives, to the other communication lines connected to a communication unit, a notification that relay of the message is prohibited.