1. An image display method of displaying an image on a monochrome display having sub-pixel structure in a main pixel comprising the steps of: transferring digital image for displaying, by way of an interface for said digital data, image expressed by a number of steps of gradation for one sub-pixel being different from that for the other sub-pixels, using image data corresponded to said main pixel of said monochrome display; and reproducing said image data corresponded to a number of steps of gradation as same as a number of steps of gradation of said one sub-pixel, using said digital data for displaying said transferred image.