Previous studies on W/O emulsion and W/O/W emulsion have reported that an appreciable amount of water permeates through an oil phase between two water phases, driven by their osmotic pressure difference. However, the variation of water droplet size during such permeation has not been quantitatively clarified. If this variation can be controlled, water droplet size and water content of a W/O emulsion will be precisely controllable over a wide range with only osmotic pressure. In this study, variation of water droplet size was examined quantitatively through two experiments using monodispersed W/O emulsions prepared by membrane emulsification. One experiment was designed to confirm the swelling of water droplets by adding pure water to an emulsion with high osmotic pressure. The other involved mixing two emulsions with different water droplet sizes and osmotic pressures. Calculated values based on theoretical transport of water through an oil phase by osmotic pressure as a driving force showed good agreement with measured values. As a result, it was found that particle size control of W/O emulsion by means of osmotic pressure is possible practically.