1. An image processing method for a diagnostic apparatus for diagnosing a disease using a captured image of an affected area, the method comprising: separating the captured image into a brightness component and a color information component; separating the brightness component into a base component and a detail component; performing a highlighting process on the detail component; restoring the brightness component from the base component and the highlighted detail component; and generating a highlighted image using the restored brightness component and the color information component; wherein performing the highlighting process comprises: acquiring the color information component that corresponds to a direction of a red-based color in a first color space; normalizing a predetermined range of the acquired color information component; and reflecting, in a highlighting coefficient of the detail component, a likelihood of a vessel of a region to be diagnosed as acquired via the normalization, to generate a highlighted detail component image.