The present invention provides a technique which makes it possible to eliminate unauthorized playback of contents where unauthorized apparatuses are used and enable only authorized playback apparatuses to play back contents properly. The playback apparatus of the present invention obtains encrypted key information that has been encrypted by a first secret-key encrypting method, and decrypt the encrypted key information into key information using a secret key stored in a storing unit. When the key information is key-updating information, the playback apparatus updates the secret key according to the key-updating information. When the key information is a decryption key, the playback apparatus (i) obtains an encrypted content key that has been encrypted by a second secret-key encrypting method, (ii) decrypts the encrypted content key into a content key using the decryption key, (iii) obtains a scrambled content that has been scrambled by a scramble encrypting method, (iv) descramble the scrambled content using the decrypted content key, and (v) plays back the descrambled content.