Abstract: To reduce the size of backup data, increase the backup speed, and solve a problem that the amount of unnecessary writing could undesirably increase in a restoration process, which would otherwise require physical disks with a capacity greater than the size of a virtual volume. On a backup server having mounted thereon a virtual volume that is the target to be backed up and restored, read/write access to only the blocks of data areas is performed by a device driver that filters I/O access to a device from a file system on the basis of the data mapping information of the virtual volume, whereby read/write access to unnecessary portions is avoided. Thus, it is possible to reduce the size of backup data, increase the backup speed, and prevent an increase in the amount of unnecessary writing to the virtual volume in a restoration process.