In a multicast delivery system, A delivery server enciphers delivery data by using a current use cipher key to generate enciphered data and transmits a multicast packet containing the enciphered data and a current use key identifier indicative of a pair of the current use cipher key and a current use decipher key as current use keys. A key management server holds as a current use key data, a set of the current use decipher key and the current use key identifier, and transmits a set of the current use decipher key and the current use key identifier as a current use decipherment key data in response to a current use key data request.