1. A contents management system comprising a plurality of subsystems which include: a storage subsystem to store a plurality of objects; a contents gateway server configured to create snapshots of the objects and store the snapshots in a first snapshot layer; a system management control unit configured to determine whether the number of snapshots in the first snapshot layer exceeds a corresponding limit for the first snapshot layer and, if yes, to migrate the snapshots from the first snapshot layer to a second snapshot layer provided by another subsystem of the contents management system, and delete the migrated snapshots from the first snapshot layer; and a contents management server configured to track location changes of the snapshots of the objects during migration of the snapshots and to re-index an index data of the snapshots of the objects based on the tracked location changes.