A storage system having a plurality of storage devices including a first type storage device and a second type storage device, a reliability attribute and/or a performance attribute of the first type storage device being different from a reliability attribute and/or a performance attribute of the second type storage device. The storage system also has a control unit and managing a plurality of virtual volumes. If necessary, a storage area allocated to a first portion of a virtual volume of the plurality of virtual volumes is changed from a first type storage area of the plurality of first type storage areas to a second type storage area of the plurality of second type storage areas while another first type storage area of the plurality of first type storage areas is allocated to a second portion of the virtual volume.