A video/audio encoding unit encodes an inputted video/audio signal in a format compliant with a predetermined technical standard. A differential signal output unit outputs the encoded data in the form of a differential signal defined by the predetermined technical standard. The differential signal output unit appends the data encoded by the video/audio encoding unit to a data transmission unallocated region of a communication line defined by the predetermined technical standard and then transmits the resulting data. Because the data transmission unallocated region, which is not directly used in normal data transmission, is thus effectively utilized in data transmission, a larger volume of information can be transmitted and received.