The present invention provides a separation method of a rare earth element and iron including: forming alkali silicate slag incorporating a rare earth element, by melting a rare earth-iron-containing material together with an alkali silicate flux in a metallic silicon melt or an iron-silicon alloy melt; and separating rare earth-containing slag from an iron-silicon alloy, in which volatilization of alkaline components contained in the flux is suppressed by performing heating and melting under an oxidizing atmosphere, and the rare earth-containing slag having a SiO2/Na2O molar ratio of 2.1 or less is formed.