Equivalent luminance contrast was investigated as an index that expresses the relationship between color difference and readability of chromatic documents. The relationships between the equivalent luminance contrast and the visual factors were determined. The visual factors are background luminance (which represents adaptation luminance in the visual field with uniform luminance), visual angle of the letters, luminance contrast, and color difference between the letters and the background. For chromatic documents, equivalent luminance contrast was found to be affected by background luminance and the visual angle of the letters. With regard to the various color combinations of background and letters, the relationship between the color difference and equivalent luminance contrast asone of the visual factors was determined. Equivalent luminance contrast was high with a large color difference. Even when there is no color difference, equivalent luminance contrast depended on the direction of the letter color from background color in the CIE1976L*a*b* color space. This directional property was almost the same for all background colors.