A disc array unit manages a shared memory as a plurality of shared memory blocks, each including a group of cache pages, performs cache control on these cache pages through use of least recently used (LRU) links, and provides an unavailable link as an LRU link to indicate that an area is not available for use as a cache page. When the shared memory block is used not as a cache memory but for another use, the shared memory block is prevented from being used as a cache memory by re-linking all the cache pages belonging to the shared memory block from an LRU link to such unavailable links individually.