The invention is directed to a storage system including a storage subsystem providing a data storage service to a host computer, and a management device managing the storage subsystem. The management device issues a command to a controller in the storage subsystem for migrating data on a migration-source volume to a migration-destination volume in accordance with a predetermined task. When the migration-destination volume is not available for use for some reason, and when the data migration is not completed normally, the management device searches an alternative volume in accordance with migration-destination requirements including essential and arbitrary requirements, and then issues a command for data migration to thus found alternative volume. With such a configuration, even if a migration-destination volume is not available for use for some reason during data migration, any other volume can be used as a migration destination in accordance with the migration-destination requirement(s).