Membrane emulsification using Shirasu porous glass (SPG) membranes is a promising technique for preparing uniformly sized emulsion droplets and has potential applications in many fields. In this study, to gain further insight into droplet formation behavior from SPG membranes in the absence of continuous-phase flow, a comparative investigation of droplet formation behavior with and without water phase flow was carried out in an oil-in-water emulsion system consisting of soybean oil and 1 wt/v % polyoxyethylene(20) sorbitan monooleate (Tween 80) aqueous solution. In the absence of water phase flow, uniformly sized oil droplets with a mean diameter of 9.88 mu m and a diameter distribution span of 0.62 were spontaneously formed from the pore openings of an SPG membrane with a mean pore diameter of 3.04 mu m. Under these conditions, the various forces acting on a droplet formed at a pore opening were estimated from the dimensionless numbers (Bond, Capillary and Weber numbers). The estimation results indicated that oil-water interfacial tension is the dominating force for the spontaneous droplet detachment, due to the fact that the oil phase distorted by the tortuous pores is transformed into spherical droplets by the interfacial tension. In the presence of water phase flow, uniform oil droplets with a mean diameter of 8.63 mu m and a diameter distribution span of 0.43 were detached from the same membrane by the shear force caused by the water phase flow before growing enough at the pore openings. The absence of water phase flow resulted in an increase in the proportion of active pores, compared to the presence of water phase flow. This is because, in the latter case, the oil droplets at the pore openings are deformed by the water phase flow, and thus the effect of steric hindrance between the neighboring droplets becomes greater.