To provide a selective separation/recovery method for a specified metal ion where, from a solution comprising two or more kinds of metal ions, only one kind of metal ion is selectively separated/recovered.

By the contact of a compound forming a metal complex such as oxalic acid, control is performed in such a manner that metal ions other than one kind of metal ion as the object for separation/recovery are made into a nondissociated state, and thereafter, only at least one kind of metal ion in a dissociated state is selectively separated/recovered by a cation exchange means. This invention is industrially useful as the method where, from an etching waste liquid produced when the indium-containing material to be etched such as an ITO film is subjected to etching treatment with an oxalic acid solution, and in which metal components such as indium and tin are present as complex compounds together with oxalic acid, the indium whose separation/recovery are extremely difficult owing to its low concentration is selectively separated/recovered.