Data to be transmitted via a serial bus in conformity with the IEEE 1394 protocol are ciphered by a ciphering/deciphering circuit, and headers are attached thereto by a header sync detecting/generating circuit. And after further attachment of CRC code by a CRC detector/generator, the data are packetized into isochronous packets of an isochronous mode by a transmission/reception switching circuit, whereby transmission of the data can be performed with enhanced security. Out of cipher keys employed, a session key invariable in each session of the data is transmitted in each packet of an asynchronous mode, and a time variable updated in each session is transmitted in each packet of an isochronous mode. And the ciphered data obtained by depacketizing the packets of the isochronous mode are deciphered, so that the data transmitted with security can be deciphered exactly, and thus illegal use of the data can be prevented with certainty.