Sugarcane yields in Minami Daito and Kita Daito islands in Japan showed a declining tendency in recent years. The cause for the reduction of sugarcane yields may be low content of humic substances. Consequently, an investigation was conducted to examine the characteristics of the humic substances, which are being considered of having close relationship with the properties of sugarcane cultivated soils. Soil samples were collected from 9 fields in Kita-daito island and 14 fields in Minami-Daito island, respectively. Soil pH and electric conductivity of the soils from both islands were in the range of about 4.0-8.0 and 0.08-0.36 mS/cm, respectively. Humic substances were extracted and quantified according to the method described by Kumada. A significant difference was recognized between humin content and humic acid content in humic substances extracted from soils of both islands. In addition, there was another significant difference between fulvic acid content and humin content in humic substances. The percentages of humic acid, fulvic acid and humin contents in humic substances were 42%, 37% and 20%, respectively. The degree of humification of humic acids in sugarcane soils showed a low degree, which was categorized as Rp type. Significant correlation coefficients were observed between exchangeable Ca and the humin in humic substances (r=0.550**), the pH and the humin content (r=0.423*), and the exchangeable Ca and the pH (r=0.863**), respectively. Experimental results revealed that organic matter (humin) accumulation in the sugarcane soils has been enhanced by the presence of exchangeable calcium. Hence, it is suggested that liming materials, which are having high concentrations of calcium can be effectively utilized to enhance the soil organic matter (humin) accumulation to improve the properties of sugarcane cultivated soils in Minami daito and Kita daito islands.