An information recording medium includes a plurality of sectors each of which has a sector header area storing a unique sector number and a data storage area for storing user data. User data to be recorded onto the information recording medium is scrambled using a data scramble key generated from a unique medium identifier of the information recording medium and a sector number of a sector into which the user data is to be recorded. The medium identifier used here has been scrambled using a medium identifier scramble key generated from predetermined data and a sector number of a sector into which the medium identifier is to be stored, and stored in that sector. In so doing, security is ensured not only for each individual information recording medium but also for each individual sector, with it being possible to protect data recorded on the information recording medium against unauthorized reproduction.