[Problem] To provide a method suitable for the industrial-scale production of a dispersion of nano-size particles. [Solution] A method of producing a nanoparticle-in-oil dispersion formed by dispersion, in an oil phase, of microparticles made up of a solid component, an aqueous liquid component, or a mixture of these, characterized by involving (1) a step for preparing a W/O emulsion-type precursor formed by dispersing, in an oil phase, an aqueous phase made up of liquid drops of an aqueous solution formed by dissolving a water-soluble substance, and (2) a step for obtaining a nanoparticle-in-oil dispersion formed by the microparticles being dispersed in the oil phase by boiling the aqueous phase in the aforementioned W/O emulsion-type precursor.
