In order to enable data migration between old and new subsystems to be performed under stopless operation, a plurality of first access paths are prepared between a CPU and an old CU (old subsystem) having an old VOL and a plurality of third access paths are set between the old CU and a new CU (new subsystem) having a new VOL. The connection is switched from the first access paths of the old subsystem as a replacement source to the second access paths of the new subsystem as a replacement destination on a plurality of occasions. When the CPU accesses the new subsystem via the second access paths on the new subsystem side during the connection change, a path replacement controller relays the access to the old subsystem via the third access paths and allows the access to be processed. Data migration from the old subsystem to the new subsystem is executed after all of the first access paths are switched to the second access paths.