Selective separation of Ga3+ was studied through a microporous polypropylene membrane impregnated with N-octadecanoyl-N-phenylhydroxylamine (ODPHA). The present membrane selectively concentrated Ga3+ from a Ga3+/Al3+ mixture against the concentration gradient, with a counter flow of H+ as a driving force. The selective separation of Ga3+ was found to be attributed to the specific interaction between ODPHA and Ga3+. A microporous membrane impregnated with ODPHA concentrated Ga3+ selectively from a Ga3+/Al3+ solution by a counter-current transport mechanism. For practical use, the selective separation of Ga3+ in the presence of a large amount of Al3+ was investigated. The separation of Ga3+ from the solution containing 50-300 fold more Al3+ was achieved, and the separation factor was evaluated to be above 300.