The present invention suitably monitors the performance of a migration destination volume and conducts data migration all over again over a short time. When conducting data migration, all data is copied from the migration source volume to the migration destination volume, and when the full copy is completed, the host access path is switched to the migration destination volume 2. The copy-pair between the volumes is maintained following completion of the full copy, and difference data created in the migration destination volume is managed. A performance monitoring unit monitors performance of the migration destination volume based on indices according to migration source volume type. When, as a result of monitoring, restoration to the status prior to data migration is decided, difference data is copied from the migration destination volume to the migration source volume, and the host access path of the host is switched to the migration source volume.