A personal computer incorporated in a numerical control device (CNC) has a function for emulating a CNC display circuit. This personal computer emulates a CNC display circuit and converts a transmitted display command A into a video signal for transmission to display means. The numerical control device (CNC) incorporating this personal computer need not to provide a display circuit for each of the CNC body and the personal computer, and, moreover, a switching circuit for selectively receiving the video signals from two display circuits and transmitting selected signal to the display means is not required either. Furthermore, the personal computer has a function for emulating a data input/output circuit of CNC. The personal computer inputs or outputs data transmitted from the CNC or data to be transmitted to the CNC, from or to a data input/output circuit in the personal computer to or from an external input/output device, through an input/output data RAM. Therefore, it is not necessary to provide data input/output circuits for both the CNC body and the personal computer.