[en] It is important to evaluate relevant processes in geosphere, such as groundwater flow, based on the actual conditions of geological environment. At the coastal area, it is needed to understand groundwater flow under the saltwater and freshwater mixing conditions. We conducted a set of laboratory experiments using mass transport visualization system with flow chamber representing homogeneous sedimentary rock filled with uniform glass beads and double layer filled with different size of glass beads, in order to observe saltwater intrusion process into fresh water and saltwater-freshwater interface behavior. From the horizontal intrusion length of saltwater, the result might not depend on the size of glass beads and might be categorized by the combination of hydraulic gradient and saline concentration. In case of low hydraulic gradient case, the saltwater intrusion length tends to be larger due to saline concentration increase. (author)