The remediation of sand contaminated with Gd-153 has been investigated using natural organic substances extracted from fallen cherry leaves instead of artificial reagents. The initial fulvic and humic acid isolated from organic substances through acidic and basic treatments have a concentration of total organic carbon of 4500 mgC/L and 3340 mgC/L, and the capacities of their functional groups are 7.23 meq/gC and 2.76 meq/gC, respectively. The remediation capacity of both fulvic and humic acid is effective in the weakly acidic region. Remediation with a natural organic substance requires a much higher carbon concentration than EDTA owing to its weak complexation with metal ions. In comparison with the EDTA complexation constant, the experimental complexation constant (log beta) of gadolinium with a natural organic substance at pH 5.5 was determined to be 10 at most.