A method of processing an image in a diagnostic apparatus 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 progression of the lesion into at least two classifications, which comprise a low stage of progression and a high stage of progression, wherein the low stage of progression is characterized by the affected area being substantially composed of black or brown color and the high stage of progression is characterized by the affected area being wholly composed of reddish vessels; and (ii) performing an image conversion process on the captured image to generate a converted image by selectively performing, based on the classification obtained in the classifying step, either a first image conversion process corresponding to the classification of the low stage of progression or a second image conversion process corresponding to the classification of the high stage of progression on the captured image, wherein the first image conversion process is a structure clarifying conversion process and the second image conversion process is a region fluorescent coloring conversion; and wherein the first image conversion process comprises a first separation step of separating the captured image into a brightness component and a color information component; a second separation step of separating the brightness component into a base component and a detail component; a highlighting step of performing a highlighting process on the base component; and a generation step of restoring a brightness based on a highlighting-processed base component and the detail component, and using the color information component to generate a highlighted image, wherein the highlighting step comprises at least one of a first highlighting step of compressing the base component in a manner brighter than a center value and a second highlighting step of performing a sharpness filter process on the base component, wherein in the generation step, the highlighting-processed base component is added to the detail component to restore the brightness, and a conversion to a second color space is performed based on the brightness restored and the color information component corresponding to a direction of a red-based color and a direction of a blue-based color in a first color space so as to generate the highlighted image. (i) classifying the captured image based on a stage of progression of the lesion into at least two classifications, which comprise a low stage of progression and a high stage of progression, wherein the low stage of progression is characterized by the affected area being substantially composed of black or brown color and the high stage of progression is characterized by the affected area being wholly composed of reddish vessels; and (ii) performing an image conversion process on the captured image to generate a converted image by selectively performing, based on the classification obtained in the classifying step, either a first image conversion process corresponding to the classification of the low stage of progression or a second image conversion process corresponding to the classification of the high stage of progression on the captured image, wherein the first image conversion process is a structure clarifying conversion process and the second image conversion process is a region fluorescent coloring conversion; and