A plurality of communication devices communicates with each other via a network in accordance with a simple network management protocol (SNMP). The communication devices include agents with specific identifications. At least one communication device includes a proxy manager. The agent updates its status log at each status transition. The proxy manager periodically monitors communication devices, irrespective of their conditions as to whether or not communication devices are each under maintenance, independently of trap messages notifying events that occur on the network. The proxy manager collects statuses of agents included in communication devices so as to store them in connection with agents. Thus, it is possible to periodically monitor agents under maintenance without conducting a status monitoring operation.