The present invention addresses the problem of providing a useful method for separating light rare earth elements and heavy rare earth elements. As a means for solving the problem, the method of the present invention is characterized by being formed by including at least: (1) a step for obtaining, from an object to be treated which includes a light rare earth element and a heavy rare earth element, a composite oxide or a mixture of oxides of both; (2) a step for dissolving the obtained composite oxide or the mixture of oxides of the light rare earth element and the heavy rare earth element in hydrochloric acid and/or nitric acid; (3) a step for adding a precipitant to the obtained solution to obtain a precipitate; (4) a step for firing the obtained precipitate; (5) a step for adding 1.1 to 3.0 times the dissolution upper limit amount of the obtained fired product to hydrochloric acid and/or nitric acid having a concentration of 0.7 mol/L or greater to obtain a solution and a residue; and (6) a step for separa
