The types of pixels of an image made up of a plurality of pixels arranged in a first direction and a second direction substantially perpendicular to the first direction are distinguished. The image is divided into a plurality of blocks, the number of pixels which are determined to be an edge pixel in each block is counted, and whether each of pixels in each block is a halftone pixel is determined on the basis of the number of the pixels which are determined to be an edge pixel in the block. When a plurality of pixels continuous in the first direction and/or the second direction are determined to be edge pixels, the number of edge pixels in each block is modified by counting as an edge pixel only a predetermined number of pixels in the set of the pixels consisting of the continuous pixels which are determined to be an edge pixel, and whether each pixel in the block is a halftone pixel is determined on the basis of the modified number of the edge pixels in the block.