When migrating data stored in a storage region assigned to a volume to another storage region, the connection status of the host computer and volume is confirmed. When the host computer and volume are connected, the maximum capacity of the volume requested by the host computer is reserved so that it is exclusively secured in another storage region to which data is to be migrated, and when the host computer and volume are not connected, the current capacity of the volume is reserved so that it is exclusively secured in another storage region.