An image processing method which includes a step for obtaining projections formed on coordinate axes having a predetermined inclination with respect to the coordinate axes of the horizontal and vertical directions. The projections are obtained run by run from image data coded in run length representation. The method also includes a step for adding the respective run-by-run projections through the entire image so as to obtain the projections of image data on the inclined coordinate axes. This method enables the image data to be processed without decoding. Further, the method allows a significant reduction in the amount of processing and also allows high-speed processing.