In a display device provided with a frame memory developing an auxiliary line image as a display image and a means of reading out and displaying the image data developed in the aforementioned frame memory, the aforementioned auxiliary line image is constituted by background pixels and dashed auxiliary lines constituted by pixels which have luminance or color that is different from that of the background color and pixels with have nearly identical luminance or color to that of the background color; and by carrying out the display of the dashes so as to move, at fixed time intervals or arbitrary time intervals, the dash display areas of the aforementioned auxiliary lines on the trajectory of the figure described by the aforementioned auxiliary lines, the system is devised to display the auxiliary lines while controlling that an identical pixel of the display device is not displayed continually.