1. A method of distinguishing the types of pixels of an image made up of a plurality of pixels arranged in a main scanning direction and a sub-scanning direction comprising the steps of carrying out a block-by-block determination in which the image is divided into a plurality of blocks each substantially larger than a visually distinguishable size, properties of the images in the blocks representing the probability that the image is a binary image are detected, probability that the image in each block is a binary image is determined on the basis of the detected properties of the image in the block, each block is determined to be a binary image block when the probability is higher than a first predetermined value, to be a tone image block when the probability is lower than a second predetermined value, and to be an indeterminate image block when the probability is not higher than the first predetermined value and not lower than the second predetermined value, and the pixels in the blocks which are determined to be a binary image block are all determined to be pixels of a binary image while the pixels in the blocks which are determined to be a tone image block are all determined to be pixels of a tone image, and carrying out a pixel-by-pixel determination on the blocks, which are determined to be an indeterminate image block by the block-by-block determination, in which properties of each of the pixels in each of the indeterminate blocks representing the probability that the pixel is of a binary image are detected, and whether the pixel is a pixel of a binary image is determined on the basis of the detected properties.