The method of anti-solvent crystallization poses problems due to high local fluctuations in supersaturation affecting the quality of the generated crystals. In order to reduce the local fluctuations in supersaturation and to achieve homogeneous crystallization fields, the utilization of a liquid-liquid interface in an emulsion is discussed. The purpose of this study was to establish the operating conditions of crystallization in order to produce fine crystals with the desired qualities by anti-solvent crystallization in an emulsion. A quaternary system of taurine/water (original solvent)/hexane (anti-solvent)/ethanol (coupled solvent) was used. As the experimental result, micro-sized taurine crystals were obtained, and the operation conditions to obtain the desired crystal shape were clarified by utilizing the proposed quaternary phase diagram.