Provide is a storage system including one or more disk drives, and one or more cache memories for temporarily storing data read from the disk drives or data to be written to the disk drives, in which: the cache memories includes volatile first memories and non-volatile second memories; and the storage system receives a data write request, stores the requested data in the volatile first memories, selects one of memory areas of the volatile first memories if a total capacity of free memory areas contained in the volatile first memories is less than a predetermined threshold, write data stored in the selected memory area in the non-volatile second memories, and changes the selected memory area to a free memory area. Accordingly, there can be realized capacity enlarging of the cache memory using a non-volatile memory device while realizing a high speed similar to that of a volatile memory device.