A control unit transfers user data and an ECC that are read from a magnetic disk and stored in an FIFO to a data buffer. An erasure position movement control unit shifts an erasure start position from a predetermined initial value at predetermined intervals of erasure movement step, and an erasure position designating unit determines an erasure correcting position based upon the erasure start position and a predetermined erasure length. An ECC processing unit performs error correction by an ECC arithmetic processing in an erasure mode on the user data and the ECC in the data buffer.