As lake sediments are one of the causes of eutrophication, their removal and effective use has been considered. When they are used as soil in a land, proper permeability is required. However, soil permeability has not completely been understood due to diversity of clays and organic compounds. We clarified permeability characteristics of Lake Kojima sediment, nonswelling clayey soil, by measuring saturated hydraulic conductivity (K). A decrease in K during percolation of dilute electrolyte solution was small when Ca was adsorbed by the soil; K became 3.4×10^-5 cm/s at pH 5, 2.9×10^-5 cm/s at pH 7, 7.5×10^-6 cm/s at pH 9, and 2.3×10^-5 cm/s at pH 11 after pure water percolation. Although the permeability tended to decrease as pH increased, K at pH 11 became larger than that at pH 9 due to chemical reaction between soil particles and Ca. On the other hand, when Na was adsorbed, K drastically decreased with pH increase; K became 7.1×10^-6 cm/s at pH 5, 3.1×10^-6 cm/s at pH 7, 1.7×10^-7 cm/s at pH 9, and 1.3×10^-7 cm/s at pH 11 after pure water percolation. The obtained results could be explained qualitatively by the theory. Next, an improvement method of permeability of Lake Kojima sediment with hydrated lime and gypsum were investigated. Three application methods were compared in the saturated hydraulic conductivity experiment; mixing, surface distribution and solution infiltration. Hydrated lime application with mixing maintained high hydraulic conductivity. The soil structure was supposed to be stabilized by chemical reaction between soil and heterogeneously distributed hydrated lime. On the other hand, gypsum application with solution infiltration maintained the same hydraulic conductivity as that of hydrated lime application with mixing. In that case, electrostatic adsorption of Ca was considered to generate flocculation and kept high permeability.