Securing redundancy for physical storage devices that are extended in units smaller than physical storage devices configuring one RAID group. When d+r pieces of physical storage devices are connected by connecting r pieces of physical storage devices, a computer: adds v×r pieces of logical chunks; adds n×v pieces of physical storage areas in each additional storage device; changes mapping information to associate n pieces of physical storage areas with v×(d+r) pieces of logical chunks under a mapping condition; in response to a write request of user data, creates redundant data; determines a first logical chunk corresponding to the write request; and respectively writes n pieces of element data including the user data and the redundant data into n pieces of physical storage areas corresponding to the first logical chunk, based on the mapping information.