Provided is a bilayer membrane vesicle capable of undergoing a phase transition. The bilayer membrane vesicle includes: (a) a fatty acid salt having 6 to 20 carbon atoms (b) an alcohol or an amine compound having an aliphatic chain of 6 to 20 carbon atoms and (c) an artificial synthetic lipid or a phospholipid capable of forming a bilayer membrane. Preferably, this bilayer membrane vesicle further contains (d) a tertiary amine as a component of the membrane. Also provided is a method of inducing a phase transition of a bilayer membrane vesicle, the method including the step of adding a dehydrating condensing agent or a dehydrating condensing agent precursor having the property of accumulating at an interface to the bilayer membrane vesicle. By causing the lipids that form a molecular aggregate to chemically change, it is possible to change the physical property and the morphology of the molecular aggregate and control the timing of phase transitions such as membrane fusion. In the membrane fusion, for example, fusion can occur without leakage of the contents of the bilayer membrane vesicle.
