1. An image processing device for fluorescence observation comprising: an image synthesizing section that generates a synthesized image by synthesizing an image signal of a reflected light image produced by illumination light obtained by illuminating body tissue with illumination light and an image signal of a fluorescent image obtained by illuminating the body tissue with excitation light; and a gain adjustment section that adjusts the gain of the image signal of the reflected light image and/or the image signal of the fluorescent image such that the boundary of the hues of the normal tissue and the diseased tissue found using the optical characteristics of the respective tissues is included in a predetermined range with respect to a prescribed standard chromaticity diagram, depending on whether the body tissue that is represented in the synthesized image generated by the image synthesizing section is normal tissue or diseased tissue.