A disk array apparatus where, when a failure occurs at a part of a cache memory, a memory area of an I/O processing controller other than the memory area where the failure has occurred is utilized without taking over the whole I/O processing to an I/O processing controller of other system is provided, so that influence of performance degradation can be minimized. In a disk array apparatus including dual cache memories, when a failure occurs at a part of the cache memory, only a memory area where the failure has occurred is closed, and reallocation thereof to another memory area of the same cache memory is conducted to continue an I/O processing.