Abstract: An optical disk, and a method and apparatus for reproducing and/or recording data to the disk are provided for preventing illegal copying of authorized disks recording copyrighted digital content. The optical disk 10 has a control area 12 for storing control data, a data area 14 for storing main digital data (content), and an identification area 13 for storing sub-digital data specific to the main digital data. The sub-digital data is recorded as a pit sequence at a locally phase modulated clock timing. When disk identification data is recorded as the sub-digital data, key information stored to the reproduction apparatus is compared with identification data (sub-digital data) detected from jitter fluctuations in the identification area 13 when content is reproduced from the optical disk 10. If a specific correlation is thus confirmed, the disk is recognized as a legally copied disk and reproduction is enabled. Illegal copies can thus be prevented.