In the analysis of data on surface X-ray diffraction, diffracted intensities are usually analyzed based on relative intensities. This requires us a lot of experimental data and theoretical calculations to discriminate the most probable model from various kinds of other models because quite different models happen to give similar intensity distributions when the number of experimental data is restricted. Furthermore, it is not easy to determine whether light atoms exist or not on the surface covered with heavy atoms. In this work, a possibility of the absolute reflectivity for determining the surface structure without such ambiguities is demonstrated.