a technique of carrying out communications between communication terminals on different lans while realizing reductions in the burden of account control and suppression in load on a server includes first and second relay servers on first and second lans issuing a registration request to a sip - server , respectively . a first communication terminal on the first lan issues a registration request to the first relay server while a second communication terminal on the second lan issues a registration request to the second relay server . the first and second relay servers exchange account information about the registered communication terminals with each other . when the first communication terminal generates communication data addressed to the second communication terminal , the first relay server selects the second relay server as a relay based on the exchanged account information to relay the communication data . then , the second relay server relays the communication data to the second communication terminal .