Provided is a method that selectively extracts and, at a low cost, recovers indium from an acidic solution containing indium and gallium. The present invention is a method that is for isolating a valuable metal and that, by means of subjecting an acidic solution containing indium and gallium to a solvent extraction that is by means of an extraction agent comprising an amide derivative represented by a general formula, extracts indium from the acidic solution. In the formula, R1 and R2 each indicate the same or a different alkyl group, R3 indicates a hydrogen atom or an alkyl group, and R4 indicates a hydrogen atom or any given group, other than an amino group, bonded to the α-carbon as an amino acid. The general formula preferably has a glycine unit, a histidine unit, a lysine unit, an aspartic acid unit, or an N-methylglycine unit.