A content reproducing apparatus includes a CPU. The CPU executes an authentication process with a memory card at a time of starting reproduction. The reproducing apparatus, when succeeding in the authentication process, can obtain from the memory card a decryption key of encrypted music data. Accordingly, by decrypting encrypted content data separately obtained by use of the decryption key, it is possible to reproduce the music data. In such the authentication process, certificate data is utilized in an order which starts from registered identification numbers and, every time failing in the authentication process, is renewed to a next identification number. Consequently, an identification number of certificate data which succeeds in the authentication process is registered and utilized in a next authentication process and the subsequent.