PROBLEM TO BE SOLVED: To provide a method for synthesizing a rare earth metal extractant capable of improving the yield of a synthesized product and the efficiency of synthetic process without using dichloromethane of a synthetic solvent in a conventional synthetic method.SOLUTION: The method for synthesizing the rare earth metal extractant comprising a dialkyl diglycol amid acid represented by general formula (1) (wherein, Rand Rare each same or different kind of alkyl, and at least one is ≥6C linear or branched alkyl) includes reacting diglycolic acid anhydride and a dialkylamine of raw materials of the dialkyl diglycol amid acid in a nonprotonic polar solvent, while regulating the molar ratio (B/A) of the dialkylamine (B) to the diglycolic acid anhydride (A) so as to be 1.0 or more, and a step for removing the nonprotonic polar solvent. The invention has a large utility value because the dialkyl diglycol amid acid having excellent ability for separating light rare earth elements can efficiently be synthesized in high yield without using the harmful dichloromethane.