1. A storage apparatus coupled to a host computer, the storage apparatus comprising: a plurality of storage devices configured with a plurality of storage tiers including a first tier corresponding to storage devices with high performance, a second tier corresponding to storage devices with middle performance, and a third tier corresponding to storage devices with low performance; and a controller configured to: define two levels of storage areas in each of the plurality of storage tiers, which correspond to a long-term storage area and a short-term storage area, provide a virtual volume including a plurality of virtual areas to the host computer; allocate a page from a pool of the plurality of storage devices to a certain virtual area of the plurality of the virtual areas according to a write request from the host computer dynamically; upon allocating a page, determine one of the plurality of storage tiers from the first tier, the second tier, or the third tier and the long-term storage area or the short-term storage area in the determined storage tier as an appropriate storage tier and a storage area, by comparing two access frequencies on a basis of virtual area of the plurality of virtual areas with a plurality of thresholds, in which the two access frequencies are measured as a long-term access frequency and a short-term access frequency; reallocate the page by migrating data corresponding to the virtual area from a current page belonging to a current storage tier and storage area to a new page belonging to the appropriate storage tier and storage area, if the appropriate storage tier and storage area is different from the current storage tier and storage area, wherein the controller is configured to: determine whether the virtual area corresponds to the long-term storage area in any of the first, second, or third storage tiers, if the determination is positive, compare the long-term access frequency of the virtual area with a first threshold of the plurality of thresholds, if the long-term access frequency of the virtual area is larger than the first threshold, compare the short-term access frequency of the virtual area with a second threshold of the plurality of thresholds, if the short-term access frequency of the virtual area is larger than the second threshold, determine the long-term storage area of the first storage tier as the appropriate storage tier and the storage area.