Readability of hardcopies as determined by image density was evaluated for the purpose of clarifying the goal of readable Electronic Paper. Subjective evaluations were carried out on the readability of hardcopies whose characters and background has various combinations of optical densities. Evaluated results are summarized as a contour map of readability on the plane of optical density: characters and background. Two conventional ways of calculating contrast ratio (OD ratio and reflectance ratio) were evaluated using the mapped values. It is confirmed that equal contrast lines as determined by OD ratio show better agreement with the contour line of readability than those based on the reflectance ratio.