1. A data migration management apparatus for managing a data migration inside a data processing system that comprises a plurality of storage areas, comprising: a memory storing an execution plan for migrating data from, a migration-source storage area, which is included in the storage areas, to a migration-destination storage area, which is included in the storage areas; a microprocessor creating, managing and executing the execution plan stored in the memory; and a communication interface for the microprocessor to communicate via a communication network with a storage controller for controlling the storage areas, wherein the microprocessor migrates the data from the migration-source storage area to the migration-destination storage area by: (1) creating the execution plan and storing the plan in the memory; (2) migrating the data from the migration-source storage area to the migration-destination storage area by issuing an instruction based on the execution plan to the storage controller via the communication network; (3) determining whether the execution plan was completed as planned or failed; (4) using the information included in the execution plan to create a re-execution plan in a case where the execution plan is determined to have tailed; (5) storing the re-execution plan in the memory in association with the execution plan; (6) issuing a separate instruction based on the re-execution plan to the storage controller via the communication network; and (7) when the execution plan is determined to have failed, determining whether or not the data migration can be retried; wherein, when it is determined that data migration can be retried, the separate instruction based on the re-execution plan is issued when the execution plan is determined to have failed, wherein, when it is determined that data migration can be retried, data migration is re-executed under same conditions as the execution plan previously performed, wherein, when it is determined that data migration cannot be retried, the migration-destination storage area is changed to another location in the memory and the data migration is re-executed.