In a relay unit, a sensor-side communication controller acquires setting information from a synchronization line and stores the acquired setting information in a storage unit in response to reception of a backup instruction from a communication master by a master-side communication controller. The sensor-side communication controller outputs the setting information stored in the storage unit to the synchronization line via a branch line in response to reception of a restoration instruction from the communication master by the master-side communication controller. The sensor-side communication controller periodically or aperiodically compares the setting information transmitted through the synchronization line with the setting information of the storage unit so as to detect presence or absence of a difference. The master-side communication controller outputs a notification signal to the communication master in response to detection of the difference.