A security module is provided in a data recording medium, data to be written to the data recording medium is encrypted with an content key different from one data to another, and the content key is safely stored in the security module. Also, the security module makes a mutual authentication using the public-key encryption technology with a drive unit to check that the counterpart is an authorized (licensed) unit, and then gives the content key to the counterpart, thereby preventing data from being leaked to any illegal (unlicensed) unit. Thus, it is possible to prevent copyrighted data such as movie, music, etc. from being copied illegally (against the wish of the copyrighter of the data).