A Relay connection unit which is connected with a plurality of buses and relays a message to a different bus. The relay connection section comprises a data check section for detecting an error in a data section of a received message, a storage section for storing set data or/and the previously sent data for message rewriting use for each ID (identifier) attached to the message, and a data rewrite portion for rewriting the data section of the message in which the error is detected into the set data or the previously sent data stored in the storage section.