To provide a cryptographic key update notifying method for simply and easily transmitting that a cryptographic key is updated between a transmission station and a receiving station.

At the time of updating the cryptographic key to be used by enciphering devices 10-1 to 10-7 by the transmission station 100, the values of the bits of the control bytes of MPEG packets 13-1 and 13-2 to be transmitted from the transmission station 100 to reception stations A to G are changed so that the update of the cryptographic key can be imparted from the transmission station 100 to the reception stations A to G.