When a primary spare area is allocated for slipping replacement and linear replacement upon initialization, and a remaining portion of the primary spare area after slipping replacement and allocated for linear replacement after initialization are insufficient, a supplementary spare area is allocated. The sizes of the primary and supplementary spare areas are determined by the number of defects generated upon initialization. The information on the sizes of the spare areas, and the remainder state information representing the degree of use of the spare areas, are recorded, so that the spare areas can be efficiently managed. Also, in the defect management method, when an area that has already been linearly replaced is allocated as a supplementary spare area, defective blocks within the allocated supplementary spare area are not used for linear replacement, and the entries of a secondary defect list (SDL) with respect to the defective blocks are not changed.