An intermediate value derived from old and new data record values or old and new data themselves are sent from a control unit to a disk unit which stores a parity record as information necessary for updating parity. The disk unit reads an old parity record and generates a new value of parity record based on the read old parity and the information received from the control unit. The generated new value is stored in an empty record on the storage medium to which a read/write head is first positioned after the generation of the new value and in which effective data has not been stored. In a disk array system, a time required to update the parity record due to the updating of the data record is reduced and a performance of the storage unit subsystem is improved.