There are provided a digital content management apparatus which further embodies a digital content management apparatus used with a user terminal, and a system which protects the secrets of a digital content. The system and the apparatus are a real time operating system using a micro-kernel, which is incorporated in the digital content management apparatus as an interruption process having high priority. When a user uses the digital content, whether there is an illegitimate usage or not, is watched by interrupting the usage process. In the case where illegitimate usage is carried out, a warning is given or the usage is stopped. The decryption/re-encryption functions of the digital content management apparatus having the decryption/re-encryption functions are not restricted to the inside of the user apparatus. By providing the decryption/re-encryption functions between the networks, the exchange of secret information between different networks is secured. By using this apparatus for converting a crypt algorithm, information exchange is made possible between systems which adopt different algorithms.