A storage system coupled to a host computer and a management device , the storage system comprising: a plurality of storage devices constituting a plurality of volumes , each of the volumes having a plurality of pool pages and belonging to one of a plurality of tiers based on a response performance; and a storage control unit for controlling the storage system, configured to - provide a plurality of virtual volumes having a plurality of virtual pages to the host computer , - allocate a pool page to a virtual page when the virtual page is first used, - construct one or more tiering groups, each of the tiering groups including one or more of the virtual volumes , - set a relocation condition to each of the tiering groups, the relocation condition including a minimum amount of use of each of the tiers , and - for each of the tiering groups, migrate the virtual pages in the virtual volumes belonging to the tiering group to an uppermost tier of the plurality of tiers until an amount of the pool pages of the uppermost tier allocated to the virtual volumes belonging to the tiering group reaches the minimum amount of use of the uppermost tier, characterized in that the storage control unit is further configured to, for each of the tiering groups, migrate the virtual pages in the virtual volumes belonging to the tiering group to a lowermost tier of the plurality of tiers until an amount of the pool pages of the lowermost tier allocated to the virtual volumes belonging to the tiering group reaches the minimum amount of use of the lowermost tier. a plurality of storage devices constituting a plurality of volumes , each of the volumes having a plurality of pool pages and belonging to one of a plurality of tiers based on a response performance; and a storage control unit for controlling the storage system, configured to - provide a plurality of virtual volumes having a plurality of virtual pages to the host computer , - allocate a pool page to a virtual page when the virtual page is first used, - construct one or more tiering groups, each of the tiering groups including one or more of the virtual volumes , - set a relocation condition to each of the tiering groups, the relocation condition including a minimum amount of use of each of the tiers , and - for each of the tiering groups, migrate the virtual pages in the virtual volumes belonging to the tiering group to an uppermost tier of the plurality of tiers until an amount of the pool pages of the uppermost tier allocated to the virtual volumes belonging to the tiering group reaches the minimum amount of use of the uppermost tier, - provide a plurality of virtual volumes having a plurality of virtual pages to the host computer , - allocate a pool page to a virtual page when the virtual page is first used, - construct one or more tiering groups, each of the tiering groups including one or more of the virtual volumes , - set a relocation condition to each of the tiering groups, the relocation condition including a minimum amount of use of each of the tiers , and - for each of the tiering groups, migrate the virtual pages in the virtual volumes belonging to the tiering group to an uppermost tier of the plurality of tiers until an amount of the pool pages of the uppermost tier allocated to the virtual volumes belonging to the tiering group reaches the minimum amount of use of the uppermost tier, characterized in that the storage control unit is further configured to, for each of the tiering groups, migrate the virtual pages in the virtual volumes belonging to the tiering group to a lowermost tier of the plurality of tiers until an amount of the pool pages of the lowermost tier allocated to the virtual volumes belonging to the tiering group reaches the minimum amount of use of the lowermost tier. <EOS>