At a seller, an encryption device encrypts content data stored in a content server using an encryption key generated by an encryption key generation device, and records the encrypted content data and a communication program on a recording medium. A paper indicating an identification code unique to each recording medium is attached to the recording medium. A decryption key corresponding to the encryption key and the identification code are registered in a managing database. A user terminal sends the identification code to a decryption key managing server apparatus via Internet in accordance with the communication program, in a case where the user terminal is to copy or install or reproduce the content data stored on the recording medium. The decryption key managing server apparatus sends a decryption key corresponding to this identification code to the user terminal. The user terminal decrypts the content data using the decryption key sent thereto.