To stabilize liquid droplets generated in a liquid-liquid interface by an electric field, within the interface.

A vessel 30 contains a first liquid layer 32 containing a first liquid, and a second liquid layer 34 containing a second liquid layer forming the liquid-liquid interface with the first liquid. An electrode 40 is provided in the first liquid layer 32, and an electrode 42 is provided in the second liquid layer 34. The electrodes 40 and 42 are counter electrodes, and respectively connected to a positive pole and a negative pole of a DC high voltage power source 50. An interface adjusting part 60 has lattice-like structure, and composed of materials easily wetted with the first liquid, and hardly wetted with the second liquid. The interface adjusting part 60 is provided near the liquid-liquid interface in the face direction of the liquid-liquid interface, with a plurality of through-holes disposed therein.