The usage amount of a pool area is avoided from becoming equal to 100%, thereby preventing all snapshot images from being crashed. There is provided a disk-array apparatus including a control processor. Here, the control processor controls a data reading/writing from/into a first logical volume created using a memory area of plural disk drives, and controls past data such that the past data is written into a second logical volume as differential data on each generation basis, the past data being stored into the first logical volume, and manages the differential data. If the amount of the differential data stored into the second logical volume has exceeded a first predetermined value, the control processor determines a generation of the differential data to be deleted from among the differential data written into the second logical volume. The control processor deletes the differential data of the generation determined.