Each time any one of HDDs is accessed, a corresponding relationship between the disk address of the accessed HDD and the time information indicating a time at which the HDD is accessed is added to a first operating-state management table. When a corresponding relationship with the same disk address already exists in the first operating-state management table, the time information is updated. When the first operating-state management table has no space available for new entry, corresponding relationships are deleted from the one having the oldest time information. Only HDDs of which disk address is recorded in the first operating-state management table is turned on.