1. An image processing device comprising: an image signal generator for generating image signals of a plurality of colors by capturing an image of a mucous membrane surface; a base image generator for generating a base image based on the image signals of the plurality of colors, the base image containing a first structure having a luminance value higher than a luminance value of a mucous membrane; a structure-extracted image generator for generating a first structure-extracted image signal by subjecting a short-wavelength image signal containing a short-wavelength component out of the image signals of the plurality of colors to a first frequency component extracting process for extracting a first frequency component so as to extract a pixel of the first structure having a positive signal value; a display controlling image generator for generating a display controlling image to be used for display control of the first structure based on the first structure-extracted image signal; and an image compositor for generating a display-controlled image in which display of the first structure is controlled by combining the display controlling image with the base image to combine the first structure of the display controlling image with the first structure of the base image.