A stored data processing apparatus includes: a format controller that adds an error correction code to data written onto a disk medium for each first block; a redundant data generation section that performs calculation for each bit position using data of all the first blocks in a second block and outputs a result of the calculation as calculation data, the second block being constituted by a plurality of the first blocks each including the error correction code added by the format controller and specified as an update target; and an MPU that writes the calculation data output from the redundant data generation section in a third block associated with the second block as the update target.