A network including a plurality of interconnecting devices, each of which interconnects communication between two transmission media and has a disconnect function for disconnecting communication to one transmission medium in a case where communication from the other transmission medium has been disconnected, is managed by storing interconnecting device information that identifies one or more of the interconnecting devices having a disconnection release function for releasing operation of the disconnect function. When a communication path formed by one or more of the interconnecting devices connected in series is disconnected, the interconnecting device information is used to determine which interconnecting devices in the communication path have the disconnection release function; an operation of each of the interconnecting devices in the communication path having the disconnection release function is sequentially controlled to acquire communication statuses of the interconnecting devices, and to diagnose a probable location of trouble in the interconnecting devices or the transmission media in the disconnected communication path; and the diagnosed probable location of trouble for the interconnecting devices or the transmission media is notified.