Provided are a preparation process of a diagnostic or therapeutic agent having a step of adding, to a first fine emulsion having a particle size of 0.5 μm or less prepared by applying a predetermined pressure to a first mixture containing a first hydrophobic compound, an emulsifying agent, and an aqueous phase, a second hydrophobic compound compatible with the first hydrophobic compound, thereby preparing a second mixture; and a step of stirring and shaking the second mixture in a hermetically sealed state, thereby embedding the second hydrophobic compound in the first fine emulsion to prepare a second fine emulsion having a particle size of 0.5 μm or less; a diagnostic or therapeutic agent prepared by the process; and an apparatus for carrying out the process.