Visible dichroism can be utilized as a measure of the degree of orientation of an amorphous region in a polymer. But the relations between dichroism and states of dyes in polymer must be elucidated before dichroism can be used more widely as a reliable means for study of amorphous orientation. To calculate the orientation factor for visible dichroism, it is necessary to decide the dichroism of an ideally oriented polymer chain. For this purpose, a single crystal of Congo Red was prepared and the dichroic spectrum of the crystal was measured. It was found that the dichroism of the Congo Red molecule itself is unity in a suitable wavelength region. As a result, a simple means for the treatment of dichroic spectra was given. The angle between the polymer chain axis and the principal axis of the absorbed dye molecule was discussed.