Halftone pixels are distinguished from non-halftone pixels in pixels making up an image according to a predetermined algorithm based on a result of edge detection for determining whether the pixels are edge pixels. The pixels which have been determined to be non-halftone pixels according to the predetermined algorithm, are continuous to the pixels determined to be halftone pixels according to the predetermined algorithm including those which have been redetermined to be halftone pixels and are not lower than a predetermined threshold density in density are all redetermined to be halftone pixels.