A disk control unit for writing and reading data to/from at least one disk device on the basis of an input/output request of an upper apparatus includes a volatile cache memory for holding data of the disk device, a non-volatile cache memory and a recording format information table stored in the non-volatile cache memory for holding recording format information of the tracks of the disk device. A write cache control unit refers to the recording format information table so as to enable the upper apparatus to perform a writing process on the volatile cache memory even when corresponding data of the disk device do not exist on the volatile cache memory.