If an I/O load on a first virtual storage area falls below a first threshold, a controller copies data in a first storage area, which is allocated to the first virtual storage area, to a third storage area in a second tier storage area, and allocates the third storage area instead of the first storage area to the first virtual storage area. If an I/O load on a second virtual storage area exceeds a second threshold which is greater than said first threshold, the controller copies data in a second storage area, which is allocated to the second virtual storage area, to a fourth storage area in a first tier storage area, and allocates the fourth storage area instead of the second storage area to the second virtual storage area.