Abstract: A mother board provided in a plural disk unit, which is formed with an interconnection pattern containing high speed control buses such as SCSI buses, is disposed at the central portion of a housing. A plurality of disk units capable of being removably inserted by plugging are directly mounted on the front side of the mother board through bus connectors, while connector units for external connection and having connectors of the control buses, terminal units having terminal circuits of the control buses, power units for supplying power to the disk units, and fan connectors having fans for cooling the interior of the housing are directly mounted on the back side of the mother board through connectors. Interconnections between the units are all contained in the interconnection pattern formed on the mother board. The entire plural disk unit is thus integrated.