1. An image processing device comprising: an illuminating portion that irradiates a subject with illumination light and excitation light; a fluorescence image-acquisition portion that acquires a fluorescence image by capturing fluorescence generated at the subject due to the irradiation with the excitation light from the illuminating portion; a return-light image-acquisition portion that acquires a return-light image by capturing return light returning from the subject due to the irradiation with the illumination light from the illuminating portion; a color-image generating portion that generates a plurality of color images by adding different types of color information that constitute a color space to the fluorescence image acquired by the fluorescence image-acquisition portion and the return-light image acquired by the return-light image-acquisition portion; and an image combining portion that combines the plurality of color images generated by the color-image generating portion, wherein at least one of the fluorescence image and the return-light image is subjected to, by the color-image generating portion, correction processing in which exponents for distance characteristics, which are approximated to exponential functions, for the fluorescence image and the return-light image are matched with each other.