In order to accurately display information on the format of different signals from respective different signal source apparatuses as an image, the output signals of analog signal sources are supplied through an interface, signal lines and other interfaces to an analog signal processing circuit. Also, output signals of digital signal sources and output signals of information generating circuits are synthesized by multiplexers and supplied to a demultiplexer through interfaces, buses and an interface. The signals from the separated signal sources are processed in a digital signal processing circuit. The signals from the separated generating circuits are supplied to an information processing circuit, so that an image corresponding to the information indicating the format of the apparatus and/or the output signal is formed, and is synthesized with the signals from the signal processing circuits to be supplied to a monitor.