Bis(phenyl hydrogen phosphonato)-1-hydroxy-3,5,5-trimethylheptane has been prepared to investigate extraction behavior of a series of trivalent rare earth metal ions. This diphosphonic acid extraction reagent exhibited high extraction ability to all ions examined, especially Sc ion. It did not exhibit apparent dependency in acid concentration, which corresponds to ion-exchange mechanism, probably due to extremely high extraction ability. Although the mutual separation of rare earths was difficult with this reagent, group separation of rare earths over divalent base metals can be accomplished. Compared with the extraction of base metals, it was found that this reagent with branched alkyl chain exhibited lower in extraction ability than diphosphonic acid with linear chain, bis(phenyl hydrogen phosphonato)-1-hydroxy-hexadecane.