PURPOSE: To surely prevent data communication for network management from being disturbed by the third person.

CONSTITUTION: A network management system 10 communicates data with a lower-order management object 14 and a higher-order network integrated management system 20 through a communication line 12. In this case, the key which is stored in a key management table 16 and is used for communication between the network management system 10 and the management object 14 is updated when the frequency in data communication between them is counted up to reach a prescribed reference value (preservation period). At this time, the new key is ciphered by the old key in the management system and is transmitted to the management object by data communication, thereby updating the key for the pertinent management object.