When data is to be written on an optical disc in the unit of a sector, an attempt is made to read a block A to which a write sector belongs for the purpose of judging whether the block can be normally read. If the block A is not successfully read, it is judged to be a defective area, and an alternative write is performed in an alternative area in the unit of a sector. The alternative write may be performed so that a plurality of sectors belonging to the same block are dispersively written in different blocks d, E. When new data is to be written in a new sector in such a situation, data is acquired from the dispersed other sectors, merged with the new data, and collectively rewritten in the same block E.