A method of reproducing an optical disk storing main digital data recorded by optically readable recording marks and sub-digital data, the sub-digital data being superposed with the main digital data and being recorded to the disk by slightly displacing positions or shapes of the recording marks in a track direction of the recording mark, the method comprising constructing a pattern based on the sub-digital data, comparing the pattern with a predetermined key information, and restricting a reproduction of content recorded by the main digital data when a correlation between the pattern and the key information is not confirmed, wherein the reproduction is restricted by dropping the transfer rate to degrade image quality, or by performing intermittent reproduction.
The key information may be obtained via a network.