The history of the liquid-liquid interface on the earth might be as old as that of the liquid. It is plausible that the generation of the primitive cell membrane is responsible for an accidental advent of the oldest liquid interfaces, since various compounds can be concentrated by an adsorption at the interface. The presence of liquid-liquid interface means that real liquids are far from ideal liquids that must be miscible with any kinds of liquids and have no interface. Thus it can be said that the non-ideality of liquids might generate the liquid-liquid interface indeed and that biological systems might be generated from the non-ideal interface. The liquid-liquid interface has been, therefore, studied as a model of biological membrane. From pairing two-phases of gas, liquid and solid, nine different pairs can be obtained, which include three homo-pairs of gas-gas, liquid-liquid and solid-solid pairs. The gas-gas interface, however, is practically no use under the ordinary conditions. Among the interfaces produced by the pairing, the liquid-liquid interface is most slippery and difficult to be studied experimentally in comparison with the gas-liquid and solid-liquid interfaces, as the liquid-liquid interface is flexible, thin and buried between bulk liquid phases. Therefore, in order to study the liquid-liquid interface, the invention of innovative measurement methods has a primary importance.