The present invention is constituted so as to enable more effective backup and restoration than in the prior art.Each time a write request is issued from an application 47, a PVOL is updated by writing the data contained in the issued write request to the PVOL 43P. In the snapshot mode, when the write frequency exceeds a frequency threshold value, the mode is switched from the snapshot mode to the journal mode. When in the snapshot mode, a snapshot of the PVOL at this point in time is acquired, either at regular or irregular intervals, for a SVOL 43S, and the snapshot in this SVOL becomes the restoration data. When in the journal mode, a journal comprising the data in a write request is written to a JVOL 43J each time the write request is issued, and a backup, to which is applied the data in a journal written to the JVOL, is acquired when executing restoration.