A suitable Tier ratio is determined according to the I/O performance and I/O characteristic of a virtual volume. A storage apparatus which is connected via a network to a host computer which requests data writing comprises storage devices of a plurality of types of varying performance; and a controller which manages storage areas provided by each of the storage devices of a plurality of types by means of storage tiers of a plurality of different types, and which assigns the storage areas in page units to virtual volumes from any of the storage tiers among the storage tiers of the plurality of types according to the data write request from the host computer, wherein the controller monitors performance information which includes I/O characteristic information for each of the virtual volumes and determines the storage tiers corresponding to the performance information thus monitored, and assigns the storage areas in page units to the virtual volumes from the determined storage tiers.