When associating a plurality of logical devices with one logical volume, the storage controller of the present invention improves the responsive performance thereof by determining the position for allocating the logical devices in accordance with the pattern of access made from the host. One logical volume is constituted by a plurality of logical devices. The control section computes the performance indicator (load) of each logical device in accordance with the pattern of access to the logical volume from the host. The control section computes the performance indicator of each RAID group for the case where each logical device is allocated on the basis of the performance indicator of each logical device. The control section determines a RAID group to allocate each logical device, so that the variance of the performance indicator of the RAID group becomes minimum.