To ensure a data by performing alternate processing on an after- defective sector causing read retrial at the time of executing a read command and based on a recovered read data by the read retrial in the midst of this read command and hence to prevent processing speed from lowering due to the read retrial and also reliability of the data from deteriorating upon access next time and afterward.

The alternate processing is performed on the sector causing the read retrial by an erasing execution part 21, a writing execution part 22 and a verifying execution part 23 in an alternate processing access range, and the sector subjected to the alternate processing is ensured in data normality by alternate processing checking part 24, and then a defective original address and an alternate address are added to an SDL, etc., by a defect list register part 25 to finish the alternate processing. Then, when the normality of the data cannot be obtained at the alternate address, a new alternate object is calculated again by an alternate address calculating part 20, and the above procedure is repeated until the normality can be confirmed.