Disclosed is a method of processing image data, including controlling to output multi-level value image data for reproducing a multi-level value image and bi-level value image data for reproducing a bi-level value image with respect to a predetermined area, thereby reproducing the multi-level and bi-level value images which constitute a single image. Also disclosed is an interface system including a multi-level converter for converting multi-level value image data into bi-level value image data and vice versa, a resolution converter operated in synchronism with the multi-level converter to convert the resolution of the multi-level value image data, an area data memory for storing area data corresponding to an instruction input representing a multi-level or bi-level value image area, and an output device for selecting the multi-level value image data from the resolution converter or the multi-level value image data from the multi-level converter on the basis of the area data and for outputting selected data to the image output device.