A monitor unit (42) monitors life or death of APL-SV (4a) and transmits the result of the monitoring to a filter instruction unit (31). The filter instruction unit (31) then stores the result of the monitoring to a life-or-death table (33). If the APL-SV (4a) is in a state of life, the filter instruction unit (31) instructs a filter processing unit (44) to perform a filter setting, while a control processing unit (34) of III-SV (3a) controls APL-CL (1a) to transmit a packet. Upon reception of the packet, a PROXY thread unit (37) of the III-SV (3a) determines whether the filter setting has been completed or not, and, if not, temporarily stores the packet into a cache (39). After completion of the filter setting, the PROXY thread unit (37) of the III-SV (3a) extracts the packet from the cache (39) and transmits the packet to the APL-SV (4a).
