When recording layers were provided on both the front and back sides of an optical disk in order to increase the recording capacity, whether the reproducing surface of the optical disk taken out of the cartridge was the front side or the back side could not be determined. This problem can be solved by adding to the BCA, in addition to disk identification information: type information indicating whether a recording surface is provided on one side or both sides; side information indicating whether the surface with which reproduction is being performed is side A or side B; recording layer quantity information indicating whether the disk has a single layer or a double layer; information indicating the areas other than the BCA and the directions in which data on the read-in, data zones, and read-out are recorded/reproduced; and information indicating the presence/absence of a disk cartridge.