At least three storage devices are connected via a network to each other. Backup data is concurrently transferred from one of the storage devices, i.e., from a copy source storage device to other ones thereof, i.e., to copy destination storage devices. The amount of data transferred from the copy source storage device to each copy destination storage device is dynamically changed according to a state of a communication connection therebetween. At occurrence of a failure in the copy source storage device, necessary data can be concurrently read from the backup storage devices to restore the data in the copy source storage device at a high speed.