For halftone digital image data, an edge portion of the image is extracted for each pixel, and based on a density of each pixel or an average density of each group of a plurality of adjacent or neighboring pixels, there are extracted pixels of a background other than the edge portion of the image. Thereafter, the image data is subdivided into a plurality of blocks each including a predetermined number of pixels. Based on distribution states of the pixels of the block judged to belong to the edge and those of the block judged to belong to the background, a domain recognition is conducted to determine whether the block is a binary block or a halftone block. In addition, for each block, a state of areas in blocks encircling the block is examined such that depending on the state, an expansion/contraction processing is repeatedly achieved a predetermined times for the halftone domain, thereby separating the image data into a binary domain and a halftone domain in a real-time fashion.