A data transfer method which eliminates erroneous operation of conventional devices not supporting encryption when copy-protected AV information is encrypted and sent on an IEEE 1394 bus. Synchronous data transferred through isochronous communication contains i) encryption identification information for indicating encryption of actual data and ii) actual data. Only the actual data is encrypted. Encryption identification information indicating encryption of actual data in synchronous data is sent together with actual data from the sending device. A receiving device detecting encryption of actual data from this encryption identification information requests decrypting information from the sending device. The receiving device decrypts the actual data using decrypting information received from the sending device according to this request.