A storage apparatus has a tiered pool including a plurality of storage tiers formed by a plurality of real page groups having different input/output performance and provides a virtual volume to which a storage area is allocated by a real page unit. At a time point before new data to be written to the virtual volume occurs based on a data object, if a real page is allocated to a write destination virtual page which is a write destination of the new data, a host computer transmits a change command to change the real page allocated to the write destination virtual page to a real page in a storage tier having predetermined performance to the storage apparatus. The storage apparatus changes the real page allocated to the write destination virtual page to a real page in the storage tier having the predetermined performance according to the change command.