PROBLEM TO BE SOLVED: To make it possible to always display the state of a computer device connected to a display device which has multiple input systems. SOLUTION: The video signals of a computer and a TV are supplied to a video amplifier 25 selectively by a selector 33 and projected by a CRT 28. The status of the computer is superposed on the video signal of the computer in a video display section and the status is decoded by a decoder 20 positioned in front of the selector 33 and supplied to a processor 23. Then OSD GEN22 generates a message display based upon the decoded status as instructed by the processor 23 and this message display is supplied to a video amplifier 25 and displayed by the CRT 28. The decoding is performed before the selector 33, so even when the TV is selected by the selector 33, the data from the computer can be always received and decoded, so that the message display can be made. Font data sent from the computer while superposed on the video signal are stored in an EEPROM 89, etc., and then a display other than fonts that the OSD GEN22 previously has can also be made.