file list l 1 the whole parallel backup operations of the nas 1 and the nas 2 will be discussed with reference to fig1 when the management client computer 8 gives the backup request to a nas cluster 100 the nas 1 serving as a host of the nas cluster 100 receives this request s 100 the nas 1 creates in the steps shown in fig5 the file list l 1 to be backed up by the nas 1 and the file list l 2 to be backed up by the nas 2 corresponding to the performance of each of the nas 1 and the nas 2 and sends these lists to the nas 2 as well s 101 then the counters a and b corresponding to the file counts of the file lists l 1 and l 2 are set on the shared logical disk 7 s 102 the nas 1 does the backup onto the backup system 9 according to the file list l 1 s 103 and decrements the file count of the backed up files sequentially from the counter a s 103 u2032 hereafter the nas 1 performs the backup by repeating s 103 and s 103 u2032 the nas 2 does the backup onto the backup system 9 according to the file list l 2 s 104 and decrements the file count of the backed up files sequentially from the counter b s 104 hereafter the nas 2 performs the backup by repeating s 104 and s 104 u2032 accordingly the files of the backup target disk drive 3 are backed up in parallel by the two storage systems such as the nas 1 and the nas 2 and hence the backup.