A data delivery system has a capability of effectively and reliably protecting copyright of the data. A video source of a movie or the like is delivered in the form of encrypted data from a server apparatus to a playback apparatus. A decryption key used to decrypt the encrypted data is stored on a storage medium such as a memory card, and the storage medium is sent from the server apparatus to the playback apparatus in parallel with the delivery of the encrypted data. After completion of the playing of the movie by the playback apparatus in a movie theater, the storage medium is returned to the server apparatus. The server apparatus examines information stored on the storage medium to check whether the video source delivered in the form of data has been properly used in an authorized manner.