According to our invention, linked operation between a monitor apparatus for displaying images, and a set-top box for transmitting image signals to the monitor apparatus via a required interface, can be achieved properly, even if both units are constructed in casings separate from each other. The monitor apparatus uses a CEC line to transmit a CEC vendor command including at least a code associated with unit information, and the set-top box apparatus transmits the CEC vendor command including at least the code associated with the unit information. Upon receiving the vendor command from the set-top box apparatus, the monitor apparatus switches to the linked operation with the set-top box.