A storage system includes a group of storage devices which include back-up devices configured to assure appropriate response time. When a data request from a host computer arrives, and the number of failed devices has changed as shown by a device state management table, a determination is made regarding the number of devices from which to read data. This determination is made based on an indication of redundancy which indicates how many of the disk devices are allowed to be in a failed state at the time of data reading. Typically, the indication of redundancy is determined by the sum of the number of failed devices and a predetermined number. The determined number of devices are selected in accordance with a selection factor, and a selection result is written into a disk management table. Then, the reading process is executed with respect to the target disk devices.