An AV system composed of an HD recorder and a display unit uses a communication interface of the HDMI. An HDMI source of the HD recorder transmits image data (image signal) in the form of differential signals to an HDMI sink of the display unit through three TMDS channels. The HDMI source inserts content identification information for the identification of the type of a content of image data to be transmitted into an AVI InfoFrame packet placed in a blanking period. A control section of the display unit controls operation of a display processing section which carries out a process for displaying for the image data based on the content identification information received by the HDMI sink and a display section for displaying an image.