For the purpose of clarifying the water pollution of Lake Kojima, two samples of water were collected and analyzed from each surface and bottom at 7 different parts in the lake. The results are summarized as follows. 1. The lake has been remarkably tainted with organic substances as grasped from the result of COD. 2. There were nutritious salts in high concentration, consequently accelerating a eutrophication. Especially, the bottom water contained a large quantity of NH(4+) because of anaerobic decomposition. 3. In summer, an overgrowth of phytoplankton was observed and pH of surface water shifted to respectable alkaline in consequence. 4. The conversion of Lake Kojima to fresh-water had not been succeeded. For the preventive measure of eutrophication, the exhaust of nitrogen and phosphorus compounds should be controled. If not so, the condition of Lake Kojima will turn to inappropriate to fish subsistence.