PURPOSE: To reduce the error rate of retrieval information without addition of a specific area and a synchronous detecting part, by adding a track number and a sector number to a real data as retrieval information to be written in an alternation sector.

CONSTITUTION: A sector divided into several sectors with optional fixed length by an optical disc consists of a sector detecting part 11, a synchronous detecting part 12, a present sector address part 13, and a data part 16. The data part 16 is composed of a real data 14 and a retrieval information 15. The track No. and sector No. to which the real data 14 is written are used as the retrieval information 15. By using said sector format, the error correction of the real data 14 and the retrieval data 15 can be performed through error correcting operation.