A cipher key is generated by first information shared in secrete between a data transmitting unit 10 and a data receiving unit 20, second information derived from duplication control information of transmit data and third information which is time change information shared between the data transmitting unit and the data receiving unit to cipher data by a CPU 12 by using the above-mentioned cipher key to transmit, from the data transmitting unit 10 to the data receiving unit 20, transmit data in which the duplication control information and the time change information are added to the ciphered data.