The power supply device of the present invention supplies power individually to a plurality of disk drives by rendering a plurality of DC/DC converters redundant. One redundant power supply substrate is assigned to a plurality of normal power supply substrates. One redundant power supply substrate supports the outputs of a plurality of normal power supply substrates. The main DC/DC converters in the normal power supply substrate correspond with the subgroups on a one-for-one basis. The secondary DC/DC converters in the redundant power supply substrate each correspond with all of the respective subgroups and are able to supply power to a predetermined single disk drive among the respective disk drives in the subgroups for each of the subgroups.