Data byte errors are detected by a comparator and classified into errors for respective interleaves, and the classified errors are counted by counters in an error count block. Further, at the time one sector has been read, comparators compare the counted values for the respective interleaves with an interleave threshold to detect whether nor not there are interleaves which have a number of errors exceeding the interleave threshold, and the detected results are counted by an error count block. The frequency of retries after an ECC is predicted from the counted value on the error count block and the total number of read sectors.