PURPOSE: To efficiently extract and separate rare earth metal ions by extracting these ions from an acid aq. soln. containing rare earth metal ions by using a solvent containing a specified alkyl phosphoric acid or its salt. CONSTITUTION: The extracting solvent is prepared by dissolving one or more kinds of compds. expressed by formula [I] [e.g. (1,3-dimethylbutyl) (2-hexyldecyl) phosphate] by 0.01-2mol/l in an org. solvent (e.g. kerosine). In the formula, R1 is a secondary alkyl having 5-12 carbon number, R2 is a branched primary alkyl having 10-18 carbon number, X is H, univalent or bivalent metal ion or NH4. The acid aq. soln. containing rare earth metal ions has 0.01-2N acid concn. (e.g. hydrochloric acid) and 10-500g/l rare earth metal ion concn. as rare earth oxide (R2O3). To effect extraction, it is generally preferable to bring the extracting solvent into contact with the acid aq. soln. containing rare earth metal ions in a counter flow at 10-50°C by using a multi-step liquid-liquid contacting device. Especially, this extracting agent has a characteristic to easily effect reverse extraction.