1. An image processing method comprising the steps of: an input step of inputting binary image data; a resolution converting step of converting a resolution of the binary image data inputted in said input step in a manner such that the binary image data for one pixel becomes data for a plurality of pixels; a detecting step of detecting a pixel, which is an isolated point, in the binary image data inputted in said input step; and an output step of outputting a light-and-dark image based upon an output obtained in said resolution converting step, wherein, in said resolution converting step, the binary image data for the isolated pixel detected in said detecting step, is diffused into a broader area than an area required for the plurality of pixels obtained in said resolution converting step.