Kawaminami Marsh, located in Kawaminami Town, Koyu County, Miyazaki Prefecture, is the only national natural treasure in Kyushu. However, precious marsh plants are threatened by urbanization around the marsh which is lowering the groundwater level and worsening the water quality due to the inflow of waste water. As the aim of this research is to preserve and increase the populations of plants in the marsh, groundwater level fluctuation, precipitation, water inflow into the marsh, and marsh water quality were surveyed and plants in the marsh were observed in the period of 1995 to 2001 of fiscal year. The surveys verified the very large influence of the water environment on preservation and propagation of aquatic plants in the marsh. In particular, the lowering groundwater level has caused by the increasing demand of water resources, and the input of waste water from the surrounding area has resulted in the eutrophication of the marsh water, and as a result, marsh plants were found to wither and die. To preserve marsh plants and encourage their growth in the future, the waterway traversing the marsh was backfilled. Cedar trees on the south side, evergreen trees on the north side, and waste farm products on the north side of the marsh were removed. These measures improved the ventilation of the marsh and also improved the sunlight and groundwater flow, which are essential for the growth of marsh plants. Due to these changes in the marsh environment, the number of marsh plants is increasing, indicating the gradual restoration of plants in Kawaminami Marsh. Measures in the marsh have been almost completed. In the future, waste water discharged from a nearby hospital will be redirected to the sewage system and the irrigation tank supplying water to the marsh will be dredged to remove nutritious sediment at the bottom.