Whether the comparison value of temporarily stored data which is read out from a flash memory by a host system exceeds a threshold value related to a bit error or not is checked, and if the comparison value exceeds the threshold value, the temporarily stored data which is read out is rewritten into the flash memory. If the temporarily stored data has an error, the error is corrected by an error correction part and then the data is rewritten. The threshold value includes, e.g., the number of readouts, the number of bit errors and the number of accumulated occurrences of bit errors. The present invention is suitable for prevention of bit errors due to read disturb and can recover the bit data which changes with time, and therefore makes it possible to improve the reliability of the flash memory by preventing occurrence of bit errors.