Two cores of bottom sediment from a small coastal lagoon Koyama-ike were examined to find out the environmental changes. KY9301 cored nearly at the center of the lake is 131cm in length. and KY9401 at the east part of the lake is 93cm. Based on the result of analyses on the water content. grain-size. total organic carbon (TOC). total nitrogen and C/N ratio. they were divided into three units. Unit A'. B'. C' in KY9301 and Unit A. B. C in KY9401 in descending order. The Unit A and Unit A' are characterized by high water-conten in spite of the coarser-grained muddy sediment. high TOC and low C/N ratio. From these data, it can be infered that during this stage the primary productivity of the lake was high with decomposition of organic matter not much advanced. In the Unit B and Unit B ' . grain-size is gradually fining upward with decreasing TOC and rather stable C/N ratio. This seems to indicate that the environment was kept in balance between the addition and the decomposition of organic matter. The Unit C and C' show cyclic changes in all analyzed data. The unit may have been repeatedly flooded with the periodic sediments.