The present invention provides a bilayer membrane vesicle capable of undergoing a phase transition, and the bilayer membrane vesicle comprises as components of the membrane, (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 comprises (d) a tertiary amine as a component of the membrane. The present invention also provides a method of inducing a phase transition of a bilayer membrane vesicle, the method comprising a step of adding a dehydrating condensing agent or a dehydrating condensing agent precursor having property of accumulating at an interface to the bilayer membrane vesicle. According to the present invention, 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 leaking out the contents of the bilayer membrane vesicle.
