An image processing device includes: one or more processors including hardware. The one or more processors are configured to: calculate, on a basis of a reference image acquired by capturing an image of a reference subject which has an optical characteristic that is equivalent to at least a part of a living body, image transformation parameters through congruence transformation that transforms a coordinate corresponding to a color of target region included in the reference image defined in a color space into a coordinate corresponding to an achromatic color in the color space; and perform, in the color space on a basis of the calculated image transformation parameters, the congruence transformation of colors of a color image acquired by capturing an image of the living body, the color image being constituted by at least two monochromatic image corresponding to different illumination having different center wavelengths.