A content stored in an information recording medium is managed on a per unit basis and protected from unauthorized use. The content stored in the information recording medium is divided into units, and the content is encrypted with a unit key generated for each unit. A unit key generation key is generated based on a variety of key generation information. The unit key for each unit is generated by applying a record seed corresponding to each unit to the unit key generation key. For example, a block key is generated based on the unit key and a block seed per block unit set for every 3 sectors. An encryption process is performed on a per block unit basis using the block key. During decryption, the decrypting of the content is permitted conditional on the matching in the key generation information, the record seed, and the block seed stored in a disk. Unauthorized use of the content is thus prevented.