A signal transmission method applicable to prevention of modification or alteration of the playback mode information, such as copying management information or charging information. The playback mode information form a terminal is data-converted for ciphering by a playback mode information appendage circuit in a header appendage circuit responsive to the key information from a terminal and the resulting information comprised of the data and the playback mode information is outputted. The reproducing side effects data conversion on the ciphered playback mode information for deciphering, by a playback mode information detection circuit in a header separation circuit, using the key information from a terminal, for taking out the original playback mode information at a terminal.