A data transfer method for preventing the malfunction of the conventional equipment which does not cope with encipherment at the time of transmitting enciphered AV information protected by a copyright through an IEEE 1394 bus. In the method, cipher identification information indicating the state of encipherment of a real data part and the real data part are contained in synchronous data transferred through isochronous communication and encipherment is effected only on the real data part. According to this method, a transmitter transmits cipher identification information indicating the state of encipherment of the real data part contained in the synchronous data together with real data and a receiver which detects the fact that the real data part has been enciphered from the cipher identification information requests the transmitter to send deciphering information and, upon receiving this information from the transmitter in response to the request, deciphers the real data part by using the deciphering information.