We previously proposed a new emulsification process, called membrane emulsification, for production of an O/W emulsion using porous glass membrane with uniform pores, and reported the conditions for preparing a monodispersed O/W emulsion and the effects of surfactant on monodispersion. In such membrane emulsification, the most important condition in addition to the uniformity of membrane pore size is that the membrane surface is more wettable by the continuous water phase than by the dispersed oil phase. In this study, we applied the membrane emulsification to the preparation of a W/O emulsion and tried to find the optimum conditions for preparing a monodispersed W/O emulsion. As with the O/W emulsion, it was found that wettability of the membrane is very important, that is, the membrane surface should be more wettable by the continuous oil phase than by the dispersed water phase. Furthermore, it was found to be an extremely important condition for preparing a monodispersed W/O emulsion that the water phase should have an osmotic pressure.