A mixed solution in which a compound which forms a nonvolatile membrane at a gas-liquid interface through self-aggregation is dispersed in a volatile liquid, is ejected and atomized in a gas or in a vacuum to produce a droplet particle, and a nonvolatile membrane is then formed at the gas-liquid interface by aligning the compound on a surface of the droplet particle having a reduced diameter due to evaporation of the volatile liquid in the gas or in the vacuum to stabilize the droplet particle. The method produces a novel droplet particle which stably exists in a gas or in a vacuum without collapsing or without evaporation of the volatile liquid inside the droplet particle.