In a dot image discrimination circuit, a dot image in an area composed of many multi-tone pixels including a central pixel is discriminated. A pixel (change point) at which the density of the image data is prominent in a first direction is detected in a plurality of pixels. Then, it is inspected if change points exist continuously in a second direction perpendicular to the first direction. The change points are counted in an area of a prescribed size around the central pixel, but some change points existing continuously in the second direction are not counted. Thus, a pixel in an image made of regular dots can be discriminated from a pixel in another type of image such as a character image. The discrimination of a dot image can be performed fast and precisely.