The method performed by the apparatus includes interconnecting a first file system managing unit 310 between the plurality of client computers 100 and the second file system managing unit 410, creating a first directory /root in a first file system managed by the first file system managing unit 310, associating a first directory /root of the second file system with the first directory /root of the first file system, enabling on-demand virtualization of the second file system by the first file system managing unit 310 based on client requests received from the client computers 100 at the first file system managing unit 310 and based on the association between the first directory of the first file system and the first directory of the second file system, and enabling indirect client access to the second file system through the first file system.