This storage system modifies the migration plan in accordance with the state of the migration destination when a plurality of volumes are migrated all at once. Migration-source volumes are migrated collectively to volumes inside the migration-destination storage apparatus. The user can make settings related to migration-source volumes and migration-destination volumes in a migration plan, and can establish a mid-process control plan for modifying the migration plan in the middle of processing. If a failure occurs in the migration-destination storage apparatus subsequent to the commencement of data migration processing, a processing method controller either cancels or temporarily halts the data migration processing, or changes the migration destination, on the basis of the mid-process control plan. When changing the migration destination, a previously selected alternate storage apparatus is selected as the new migration-destination storage apparatus. When a failure occurs in the alternate storage apparatus, yet another alternate storage apparatus is selected.