As shown in FIGS. 2A and 2B, boards and units for offering various types of functions to the disk array apparatus 10 are mounted in the basic casing 20 and additional casing 30. [0030] As shown in FIG. 2A, a plurality of disk drive units 52 each having a disk drive 51 provided therein are mounted as arranged in a row on the front upper stage of the basic casing 20.