The present invention relates to a security management system of a computer network, which includes a center server and two or more relay servers. The relay servers receives at least some of data stored in the center server and stores the received at least some of data. A first relay server stores access authentication information and transmits data requested by the client to the client, when access information received from a client does not match with the access authentication information. The center server transmits a ‘block relay’ command to the first relay server and a ‘start relay’ command to a second relay server, when the center server receives information on the malicious access. Accordingly, the second relay server performs a relay function instead of the first relay server.