In a system for storing data as a backup purpose to a separate storage apparatus, data of a copy source is matched with data of a copy destination in a predetermined time. A host computer attaches an identifier to a write request and then transmits the resulting write request to another storage apparatus for every segment of business. The host computer calculates an amount of data which can be copied (backup) within a given time for every identifier, and then, issues a backup instruction to the storage apparatus.