Phosphonylation of kojic acid has been achieved using inorganic diphosphonate (DP) in aqueous solution. The optimum condition for the phosphonylation of kojic acid with DP is kojic acid : DP = 0.1 mol/L : 0.5 mol/L, pH 9 and 25 ºC. 2-Phosphonylmethyl-kojic acid was synthesized with the yield of 45 %. The reaction mechanism of kojic acid with DP was discussed. We have successfully introduced the phosphonate group in kojic acid. Phosphonylated kojic acid is expected that the permeability of kojic acid into the skin will be increased by introducing a phosphonate group.