1. An image processing device comprising: an image signal input unit which inputs image signals of three colors; a base image creation unit which creates a base image based on the image signals of three colors; a signal ratio calculation unit which calculates a first signal ratio between the image signals of two colors among the image signals of the three colors and a second signal ratio between the image signals of two colors different from the colors for the first signal ratio; a frequency enhancement unit which generates a frequency component-enhanced image, in which a frequency component corresponding to an abnormal part different from a normal part is enhanced, based on the base image; a composition ratio setting unit which sets a composition ratio indicating the ratio for composing the frequency component-enhanced image with the base image based on the first signal ratio and the second signal ratio; and a composition unit which composes the frequency component-enhanced image with the base image at the composition ratio set by the composition ratio setting unit to generate a structure-enhanced image, in which the structure of the abnormal part is enhanced.