The invention provides a method of processing an image in a diagnostic apparatus 100 of diagnosing a lesion using a captured image of an affected area to be diagnosed, comprising the steps of: (i) classifying the captured image based on a stage of a progression of the lesion (Step S13 and S14); and (ii) performing an image conversion processing, which corresponds to a classification obtained as a result of step (i), on the captured image to generate a converted image (Step S15-17).