A compound storage system including: a storage box having a plurality of storage devices; and a plurality of servers capable of executing one or more virtual machines. The storage box stores a logical volume. The virtual machines executable by the server include an application VM and a controller VM. When a predetermined situation occurs in which an application VM of a migration source server is migrated to a predetermined migration destination server, at least one processor of the one or more servers in the compound storage system migrates the application VM to the migration destination server, and migrates a control right of a logical volume used by the application VM to a controller VM of the migration destination server.