Soils of the slope on the southern side of the Sansyou-uo swamp in the Institute for Nature Study were the hydrocatena, of which Ao horizon corresponded to the hydro-condition caused by the relief. Humus composition and sugar composition of these soils were investigated, and discussed about the accumulation form of the soil organic matter. Organic-carbon, nitrogen and sugar contents of these soils were high. Especially, those of soil located on the upper part of the slope were higher than those of soil located on the lower part. Degree of the humification of humic acid was lower, and the percentages of component sugars mainly derived from plant among all component sugars were higher. Of the soil located on higher part of the slope, hydro-conditdon was drier, and the acidity and the amount of the exchangeable cation were lower. It is considered that the high accumulation of the organic matter was caused by the low rate of decomoposition of the organic matter such as plant residue, because of the condition described before. But it could not explain that the high contents of the orgasic carbon and the darkness of all soils from the low rate of the decomposition. It is suggested that the some portion of the soil organic matter was survived which was accumulated under the past condition such as grass vegetation. The organic carbon content was corresponded with the depth of the Ao horizon. It is suggest-ed the similarity of the decomposition of organic matter in Ao and A horizon.