Abstract: A storage apparatus with which billing amounts in the case of write count-dependent billing can be predicted, comprises a physical storage device which provides logical volumes of different types, and a controller which executes I/O control by classifying each of the logical volumes of different types into tiers of different response performances. In a case where a write request is issued to any virtual page which configures the virtual volume, the controller assigns any real page which configures a logical volume to the virtual page, writes data to the real page, and performs a write count. The controller calculates a billing amount per fixed time interval on the basis of the performed write count, calculates tier ranges so that the billing amount per fixed time interval does not exceed the billing upper limit value, and relocates the real page on the basis of the calculated tier range.