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 15P is data-converted for ciphering by a playback mode information appendage circuit 14 in a header appendage circuit 13 responsive to the key information from a terminal 15K 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 26 in a header separation circuit 25, using the key information from a terminal 27K, for taking out the original playback mode information at a terminal 27P.