PURPOSE: To provide the mutual separation method for a lanthanoid element and a trans-plutonium element (TPE) selectively extracting the trans-plutonipm element without generating a waste liquid, compatible with the continuous process, and using an inexpensive chemical.

CONSTITUTION: An organic solvent. containing an N-heterocyclic compound and hydrophobic anions is added to an acid aqueous solution containing a lanthanoid element and a trans-plutonium element, and the trans-plutonium element is extracted in the organic phase to separate the lanthanoid element and the trans-plutonium element.