A stored data processing apparatus comprises: a calculation section that performs calculation for each bit position using data of all first blocks in a second block that has been specified as an update target on a storage medium and outputs a result of the calculation as calculation data; a write section that writes the calculation data in the third block associated with the update target; and a read section that reads out data from the first block specified as a read target. In the case where it is determined that a plurality of data can be read out from the read target, the read section sets the read target as a duplication block, reads out data from the first blocks other than the duplication block which belong to the second block including the read target so as to set the read out data as determination data, reads out data from the third block corresponding to the read target so as to set the read out data as redundant data, sets one of the plurality of data that can be read out from the duplication block as selection data, and determines whether the selection data is correct data using the determination data, redundant data, and selection data.