To enhance operability and usability for a user of a computer and to prevent the occurrence of malfunctions of a display device by careless operations.

Control information of picture display incorporated in a keyboard and software is transmitted to a display device 6 via a communication control circuit 5 after processing it with a CPU 2 in a computer body 1. A microcomputer circuit 7 in the display device 6 takes out the control information from a communication control circuit 8 and controls the prescribed places of a video circuit 11 and a deflection circuit 10. In this case, the computer body 1 first transmits identification information to the display device 6 and permits the control when the identification information coincides with the identification information to be stored in a memory circuit 9.