A diagnostic apparatus for diagnosing a disease using a captured image of an affected area, the apparatus including a memory configured to memorize the captured image; and a processor configured to process the memorized image memorized in the memory. The processor is configured to perform a highlighting process which includes separating the captured image into the brightness component and a color information component; separating the brightness component into a base component and a detail component; highlighting the separated brightness component; and adding a region to be diagnosed in the captured image to the highlighted brightness component while a specified color of the region to be diagnosed is maintained.