When there is a change in a group of volumes managed by a host computer, data duplication processing is immediately carried out against the changed volume. The host computer includes a volume-managing portion, a data duplication-controlling portion which executes the data duplication of data stored in a volume in a main data center, and a data duplication storing portion which stores data necessary for the data duplication. The data duplication-controlling portion compares data held by the volume-managing portion with the data in the data duplication storing portion, and updates the data in the data duplication storing portion based on the data held by the volume-managing portion.