To quickly resume data migration by properly monitoring the performance of a migration destination volume.

When data migration is performed, all data are copied from a migration source volume 1 to a migration destination volume 2 (S1), and the access path of a host 4 is switched to a migration destination volume 2 after the completion of full copy (S2). Even after the completion of full copy, a copy pair between respective volumes 1 and 2 is maintained, and a difference to be generated in the migration destination volume 2 is managed (S3). A performance monitor part 6 monitors the performance of the migration destination volume 2 based on an index corresponding to the type of the migration destination volume 2. When it is decided that a status before data migration should be restored as the result of monitoring (S4), difference data are copied from the migration destination volume 2 to the migration source volume 1 (S5), and the access path of a host 1 is switched to the migration source volume 1 (S6).