Multiple types of storage devices which have different performance are appropriately allocated to multiple virtual volumes in accordance with the performance requirements of the respective virtual volumes. In cases where, among virtual volumes 82 for which response times have been specified, there is a [virtual volume] that has a shortage of pages to which SSDs 70 should be allocated and that does not satisfy the performance requirement when its Tier boundary value β is adjusted to Tier boundary value β′ and, if unallocated pages as the pages to which SSDs 70 should be allocated do not exist, Tier boundary value γ or Tier boundary value α of another virtual volume is adjusted to Tier boundary value γ′ or the Tier boundary value α′, and pages to which SSDs 70 are allocated are secured in the virtual volume 82 for which the Tier boundary value has been adjusted.