A method of selecting logical volumes that are the targets for data migration to equilibrate the load on a system, based on the accessing data of the physical drives and logical drives under the disk array controllers, without increasing the load of the disk array controller. An external manager communicates with two or more disk array controllers, gathers and manages the access data and the configuration data relating to the physical drives and logical volumes of each disk array controller, and prepares an optimum data migration instruction to equilibrate the access load.