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This paper proposes a method for illumination-invariant representation of natural color images. The invariant representation is derived, not using spectral reflectance, but using only RGB camera outputs. We suppose that the materials of target objects are composed of dielectric or metal, and the surfaces include illumination effects such as highlight, gloss, or specularity. We preset the procedure for realizing the invariant representation in three steps: (1) detection of specular highlight, (2) illumination color estimation, and (3) invariant representation for reflectance color. The performance of the proposed method is examined in experiments using real-world objects including metals and dielectrics in detail. The limitation of the method is also discussed. Finally, the proposed representation is applied to the edge detection problem of natural color images.