A RAID-group converting apparatus converts two RAID groups into one RAID group. The RAID-group converting apparatus includes: a data-reading unit that reads at least non-parity data from data including the non-parity data and parity data, for each stripe from a plurality of recording media belonging to the two RAID groups; a parity-generating unit that generates two parities using the data belonging to a same stripe read by the data-reading unit; and a parity-writing unit that writes the two parities generated by the parity-generating unit into a parity storage area belonging to the same stripe.