PURPOSE: To recover by separation an alkali-metal ion with higher atomic number by occlusion in a compound mentioned below, by subjecting an iron cyanide compound to electrolytic reduction in an aqueous solution containing several alkali-metal ions.

CONSTITUTION: A platinum-plate electrode on which the iron cyanide compound such as Prussian blue etc., is electrodeposited is dipped in the aqueous solution containing several alkali-metal ions, to which a reduction current is sent by use of a platinum wire as a counter electrode to carry out electrolytic reduction. Subsequently, the above platinum-plate electrode is dipped in another aqueous solution free of alkali ions, to which an oxidation current is sent by use of a platinum wire as a counter electrode to carry out electrolytic oxidation. In this way, the alkali-metal ion with higher atomic number occluded in the iron cyanide compound at the electrolytic reduction is eluted in the above- mentioned different aqueous solution so as to be recovered by separation.