Sayamaike Pond is the existing oldest pond in Japan. The excavations at this site carried out prior to the latest improvement project proved that the pond had been built about 1.400 years ago. The section of its embankment consists of the earth layers, which clearly showed the history of the improvement and the civil engineering technologies of ancient times. In view of the historical importance, Osaka Prefectural Government decided to preserve the entire section. Here I introduce you how the preservation work had been completed.