A contents database in which data of contents enciphered by a C key is stored and provided for a contents server, wherein the contents data enciphered by the C key are enciphered by an M key and sent to a user machine. In the user machine, the contents data enciphered by the C key are stored in a storage device. Upon reproduction, the contents data enciphered by the C key from the storage device are sent to an enciphering/decoding processing unit and decoded and charging is performed in accordance with the C key. A dynamic authenticating code which dynamically changes with the elapse of time is added to the C key.