In an optical disk, a substrate has main data recorded in the form of a combination of pits and lands on one side thereof, and at least a reflective layer and cover layer are stacked on the substrate to cover the pits and lands on the substrate. To the optical disk, sub data is recorded in the form of marks formed by irradiating writing-power laser light to the reflective layer. The marks are formed so that reading signal level will be raised where the marks are formed while it will be lowered where the marks are formed in a counterfeit disk produced based on an authenticated disk, namely, the reading signal level where the marks are formed is different in polarity between in the authenticated and counterfeit disks. Further, in a player for the optical disk, discrimination is made between the authenticated and counterfeit disks by detecting the sub data recorded in the optical disk and judging whether the value of the detected sub data has been read at a correct polarity.