Upon reception of a query about the address of a server from a client, a DNS server sends a query about the address of that server to an external DNS server. The DNS server checks based on the address obtained from the external DNS server if connection to the server can be established. If connection to the server cannot be established, and the DNS server receives a query about the address of the server, the DNS server returns a response indicating that the address is not available. When the address is returned before the check process, the DNS sever sets an expiration time shorter than that of the address of the server obtained from the external DNS server as the expiration time of the address of the server.